Questions

Foundational Virology — Questions

Study questions for the Foundational Virology topic — exam-style, clinical-scenario and FAQ.

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97 questions: 97 MCQ, 0 written.

High-priority (past-paper) questions only

Most particles never form a plaque. This is also why viral load (genome copies measured by PCR) is not the same as infectious titre: PCR cannot distinguish a whole virion from an empty shell.

Positive-sense RNA has the same sequence as mRNA, so a ribosome translates it immediately, producing the polymerase the virus then uses to replicate.

Positive-sense RNA has the same sequence as mRNA and is translated directly (picornaviruses, flaviviruses, coronaviruses). Negative-sense RNA must first be transcribed by the virion’s own RNA-dependent RNA polymerase (RdRp).

Retroviruses replicate through a DNA intermediate using reverse transcriptase (an RNA-dependent DNA polymerase) rather than an RdRp. The hepadnaviruses (hepatitis B) use the same enzyme, though they are DNA viruses.

Priming cleavage does not switch fusion on; it leaves the fusion protein metastable (“primed but waiting”). The trigger (low endosomal pH for influenza, or receptor binding for others) then releases it into its fusion-active form.

The hepadnavirus (HBV) genome is a partially double-stranded circle repaired to cccDNA in the nucleus; this persistent template is a major reason HBV is so hard to cure.

Dolutegravir (and the long-acting cabotegravir) are integrase strand-transfer inhibitors that block insertion of the reverse-transcribed HIV DNA into the host genome.

The two are unrelated concepts that happen to share a word. The taxonomic class is one rung of the 15-rank hierarchy (suffix -viricetes); a Baltimore class groups viruses by the route from genome to messenger RNA.

Naked viruses generally accumulate and are freed when the cell bursts; enveloped viruses wrap themselves in host membrane as they bud out.

Haemagglutinin tethers budding virions to surface sialic acid; neuraminidase cleaves it to free them. Blocking neuraminidase traps progeny at the surface and halts spread.

The vernacular name denotes the agent; the binomial denotes the taxonomic species. Vernacular virus names are unregulated by the ICTV.

The RNA polymerases of RNA viruses do not proofread and make errors roughly ten thousand times more often than DNA polymerases, generating the quasispecies swarm that escapes immunity and drugs.

They are chain-terminating nucleos(t)ide reverse-transcriptase inhibitors, active against HIV reverse transcriptase (and the HBV polymerase). Nevirapine, by contrast, is a non-nucleoside RT inhibitor that binds the enzyme allosterically.

Reassortment of segments between two influenza viruses in a single cell produces an abrupt, large antigenic change (shift), distinct from the gradual point-mutation drift.

Baltimore sorts viruses into seven classes by the route from genome to messenger RNA. It is non-hierarchical and complements, rather than replaces, ICTV taxonomy.

Realm, kingdom, phylum and class were added, all sitting above order. The basal rank is “realm”, not “domain”.

These are protease inhibitors. A virus that translates its genome as one polyprotein must cut it into functional units with a viral protease; blocking that protease leaves the polyprotein uncut and the virions immature.

Because no host enzyme copies RNA from RNA, an RdRp inhibitor can be highly selective for the virus. Sofosbuvir (hepatitis C) and remdesivir (SARS-CoV-2) act here.

A virus can only infect cells that display the receptor its ligand binds, so receptor distribution largely dictates which tissues are infected and what disease results.

The 15-rank, Linnaean-like hierarchy was ratified in two ICTV votes, in 2018 and 2019.

Lipid solvents and detergents dissolve the envelope and destroy the infectivity of enveloped viruses while leaving naked viruses intact. This is the basis of the ether-sensitivity test for an envelope.

Only icosahedral and helical symmetry occur. Particles that are neither (poxviruses, the HIV-1 cone, tailed bacteriophages) are termed “complex”.

Retroviruses are uniquely diploid, packaging two copies of their positive-sense ssRNA genome; every other viral genome is haploid.

A polythetic class is defined by multiple shared characters, none individually required, which is why species rest on multiple criteria and tolerate within-species variation.

Unlike cellular life, viruses do not descend from a single common ancestor; the system reflects several separate roots, so there is no single virosphere-wide tree.

Species names are binomial: a genus name plus a single-word epithet, mirroring the Linnaean convention used across biology.

Only genus and species assignment is obligatory; higher ranks are populated only where the evidence justifies them, so most viruses are not placed at all 15 levels.

The ICTV defines a species as a monophyletic group distinguished by multiple criteria, set per family by the relevant Study Group.

“Arbovirus” requires genuine replication in the arthropod vector; passive mechanical carriage (the “flying pin”) does not qualify. Arboviruses span several families.

The measles species is Morbillivirus hominis (genus plus one-word epithet, italicised, genus capitalised). “Measles virus” is the vernacular agent name; “Measles morbillivirus” is a superseded transitional form.

Rabies virus (a rhabdovirus) has a helical, enveloped nucleocapsid; the others are icosahedral. In vertebrate viruses, helical nucleocapsids are always enclosed within an envelope.

Realm = -viria. (Order = -virales, family = -viridae, subfamily = -virinae, genus = -virus, phylum = -viricota, class = -viricetes.)

The three primary criteria are the genome (type, character, sequence), the replication strategy, and the virion structure. Options naming virion size or serology are the older, pre-molecular criteria.

The envelope is taken from a host membrane during budding, so its lipids are host-derived and vary with the budding site, while the glycoprotein spikes embedded in it are virus-coded.

No host enzyme can copy RNA from an RNA template or translate a negative strand, so the negative-sense (class V) and double-stranded RNA (class III) viruses must bring their own polymerase. A positive-sense genome is translated directly on entry, so its polymerase is made on the spot.

Eukaryotic ribosomes do not reinitiate at downstream genes, so viruses reach their other proteins by polyprotein cleavage, splicing, segmentation, ribosomal frameshifting, or an internal ribosome entry site (IRES).

Chloroquine raises endosomal pH and so blocks the cathepsin-L endosomal entry route used by the kidney cells; airway cells expressing TMPRSS2 cleave the spike at the plasma membrane and fuse there, never depending on the endosome.

RNA genomes sit just below an error threshold that caps them near 30 kilobases; coronaviruses are the exception, with an nsp14 proofreading exonuclease. Pushing the rate past the threshold drives the population to extinction.

Poliovirus, coxsackievirus, hepatitis A and hepatitis E can exit cloaked in host vesicles; circulating hepatitis A is entirely quasi-enveloped, which shields it from antibody.

Aciclovir is monophosphorylated only by viral thymidine kinase (HSV/VZV); the activated drug then chain-terminates the viral DNA polymerase, so it is inert in uninfected cells. Ganciclovir works analogously for CMV, activated by the CMV UL97 kinase.

M2 is a proton channel that acidifies the virion interior to allow uncoating; amantadine and rimantadine block it. Near-universal resistance has retired them clinically, but they remain the classic uncoating inhibitor.

An IRES is a structured RNA element that recruits the ribosome internally, one of several ways viruses bypass the rule that a eukaryotic ribosome reads only the first gene of a message.

Diploid strains senesce at the Hayflick limit as telomeres shorten; continuous lines such as HeLa and BHK-21 are immortalised and divide indefinitely but have drifted from normal tissue.

The dsRNA viruses (Reoviridae, Birnaviridae, Picobirnaviridae) all carry segmented genomes; rotavirus, a reovirus, has 11 segments.

Eggs, used since the 1930s, remain the workhorse for influenza virus stocks and much influenza vaccine manufacture.

On a B cell gp42 binds an HLA (human leukocyte antigen) class II molecule as coreceptor, after gp350/220 has attached to CD21, and the gH/gL heterodimer with gB then drives fusion.

This underlies the tropism switch: epithelial cells, which lack CD21, are entered without gp42 (using integrins and other proteins), so virus made in a B cell carries little gp42 and infects epithelium well, while virus made in epithelium carries gp42 and infects B cells well. CD21 is the attachment receptor, not the gp42 coreceptor.

For many virus–disease links, “risk factor” describes the relationship better than an absolute “cause”: disease appears in only a fraction of the infected, and a syndrome may have several causes.

These are informal categories defined by how a virus spreads and where it replicates; one family can appear in several, and they cut across the formal taxonomy.

HBV (class VII) is a DNA virus that copies a pregenomic RNA intermediate back into DNA by reverse transcriptase, which is why reverse-transcriptase inhibitors such as tenofovir treat it.

Ribozyviria contains small circular RNA agents with self-cleaving ribozymes, including hepatitis delta virus.

The protease normally cleaves the Gag-Pol polyprotein during maturation; inhibiting it leaves the particle assembled but immature and non-infectious.

LMP2A delivers a tonic signal resembling that of the B-cell receptor, keeping the latently infected B cell alive and blocking the differentiation that would otherwise trigger reactivation, so the cell survives without antigen.

The CD40 mimic is LMP1; the bcl-2 homologue is the separate viral protein BHRF1; the episome is tethered by Epstein-Barr nuclear antigen 1 (EBNA1); a viral DNA polymerase belongs to the lytic cycle.

Epstein-Barr virus is a gammaherpesvirus and the prototype of the genus Lymphocryptovirus; its current species name is Lymphocryptovirus humangamma4.

The other human gammaherpesvirus, Kaposi sarcoma-associated herpesvirus, sits in the genus Rhadinovirus. The alphaherpesviruses (herpes simplex, varicella-zoster) and betaherpesviruses (cytomegalovirus, human herpesviruses 6 and 7) are different subfamilies.

The linear double-stranded DNA genome (about 172 kilobases) circularises on entering the cell into a chromatin-coated episome that the viral protein Epstein-Barr nuclear antigen 1 (EBNA1) tethers to the host chromosomes; the host DNA polymerase then replicates it with the cell. It is not integrated.

Integration, a cytoplasmic linear genome, and an RNA provirus made by reverse transcriptase all describe other viruses; a viral DNA polymerase operates in the lytic cycle, not in latency.

Human strains prefer alpha-2,6-linked sialic acid (upper airway); avian strains prefer alpha-2,3 (avian gut and deep human lung). A shift in this preference is one of the features watched in pandemic surveillance.

The genus (and subgenus) suffix is -virus, as in Morbillivirus within Morbillivirus hominis.

Whole-genome sequencing is a modern criterion, not one of the historical five (nucleic-acid type, virion size, morphology, stability, antigenicity).

The receptor is the host-cell molecule; the ligand is the viral molecule that binds it. For example, the SARS-CoV-2 spike is the ligand and ACE2 the receptor.

Riboviria gathers RNA viruses replicating via an RNA-directed polymerase together with the reverse-transcribing viruses; HIV-1 sits in Riboviria, kingdom Pararnavirae.

Sub-species labels are clinically and epidemiologically important but fall outside the ICTV’s remit; conventions vary from virus to virus.

Lenacapavir binds the HIV capsid protein, disrupting both proper capsid assembly/maturation and the incoming capsid’s uncoating and nuclear import.

Letermovir blocks the terminase complex that cuts concatemeric DNA and packages it into preformed capsids, an assembly/packaging step distinct from the DNA polymerase that ganciclovir inhibits.

Maraviroc is a CCR5 antagonist: it occupies the co-receptor so gp120/gp41 cannot complete entry. People homozygous for the CCR5-Δ32 deletion are naturally resistant to CCR5-tropic HIV for the same reason.

Most DNA viruses replicate in the nucleus (poxviruses are the cytoplasmic exception); most RNA viruses replicate in the cytoplasm (influenza is a nuclear exception).

Vertex capsomeres bonded to five neighbours are pentons (pentamers); those on faces and edges, bonded to six, are hexons. An icosahedron has twelve vertices, hence twelve pentons.

Some enveloped viruses fuse directly at the cell surface at neutral pH; others such as influenza require the low pH of the endosome to trigger fusion.

Since 2018 every species must belong to a genus, a requirement not previously enforced. The type-species concept, by contrast, was abolished in 2021.

After uncoating and before the first progeny are assembled, no infectious particle exists, so none can be recovered even by breaking the cells open.

Hepatitis A, B, C, D and E belong to five unrelated families; the grouping is by target organ, not taxonomy.

The 2009 H1N1 virus was a reassortant carrying segments from four ancestral lineages: North American swine, North American avian, human, and Eurasian swine influenza.

A realm groups viruses by deeply shared genes and proteins, not descent from one ancestor, so it deliberately avoids the single common ancestry that “domain” implies for cellular life.

The binomial mandate was ratified in 2021 and essentially completed at the February 2025 ratification vote, so almost all species names are now binomial.

The mature HIV-1 capsid is a fullerene cone: a lattice of CA hexamers closed by exactly twelve pentamers, curved into a cone. Neither icosahedral nor helical, so “complex”.

Nucleocapsid = the capsid plus the nucleic acid it encloses. Where present, an envelope is added around it to form the complete virion.

HIV is a retrovirus, class VI: a positive-sense ssRNA genome reverse-transcribed to DNA and integrated. Hepatitis B is class VII (dsDNA, reverse-transcribing).

Sequestering replication concentrates the enzymes and conceals double-stranded RNA, the danger signal that RIG-I and MDA-5 detect to trigger interferon.

A sequence compared against reference genomes places an isolate in a taxon almost immediately, so partial sequencing is often the earliest step.

Why E

As a betaherpesvirus, CMV is latent in the myeloid lineage: in CD34 bone-marrow progenitors and the monocytes derived from them. Reactivation accompanies differentiation into macrophages and dendritic cells, which is why it reawakens during inflammation and immunosuppression.

For contrast, the alphaherpesviruses (herpes simplex, varicella-zoster) are latent in sensory ganglion neurons, and Epstein–Barr virus is latent in memory B lymphocytes.

The non-glycosylated matrix protein (for example influenza M1) lines the inner face of the envelope, conferring rigidity and bridging the envelope and the nucleocapsid.

The ICTV is the single body that sets viral taxonomy and the naming of taxa. NCBI and GenBank host sequence data but do not classify viruses.

Epstein-Barr virus attaches to B cells when gp350/220 binds CD21, the complement receptor 2 that normally recognises the C3d fragment of complement; gp42 then engages an HLA (human leukocyte antigen) class II molecule as coreceptor.

The other options are receptors used by unrelated viruses: CD4 by HIV, CD46 by measles, ICAM-1 by major-group rhinoviruses, and CD155 by poliovirus.

The major envelope glycoprotein gp350/220 attaches the virus to the B-cell surface by binding complement receptor 2 (CD21).

gp42 then acts as the coreceptor-binding protein, and gB with the gH/gL heterodimer drives membrane fusion. LMP1 is a latent membrane oncoprotein, not a virion attachment protein.

Oncogenic potential is recognised in Herpesviridae, Adenoviridae, Papillomaviridae, Polyomaviridae, Hepadnaviridae, Retroviridae and Flaviviridae, but not the Picornaviridae.

Viral “late-domain” motifs recruit the host ESCRT machinery to perform the final membrane scission. Influenza is the exception, severing itself with its M2 protein.

Duplodnaviria gathers double-stranded DNA viruses sharing the HK97-fold capsid (herpesviruses, tailed bacteriophages). Varidnaviria is defined instead by the jelly-roll fold.

Formal taxon names are capitalised and italicised with the rank label preceding (the genus Morbillivirus); vernacular names are lowercase, not italicised, and set informally outside ICTV control.

Naked enteric viruses (rotavirus, enteroviruses) resist stomach acid, fitting their faecal-oral spread, whereas enveloped viruses are inactivated at pH 5–6. Enveloped viruses are also the more heat-labile, −70 °C preserves infectivity for years, and prions resist boiling.

VZV establishes lifelong latency in neurons, and not only in the dorsal-root sensory ganglia but also in cranial-nerve, autonomic and enteric ganglia.

In the latent state the virus shuts down almost all gene expression, restricting itself largely to a non-coding latency-associated transcript that suppresses ORF61 and so blocks the cascade that would otherwise drive replication.

Cryo-EM now routinely achieves near-atomic resolution and has become the dominant technique for virus particles and their protein machines; X-ray crystallography is reserved for small viral enzymes and antibody-fragment complexes.

In latency the genome persists as a quiet episome expressing only a restricted set of genes, with no virus produced; this is the state of the lifelong memory B-cell reservoir. The lytic cycle, triggered by the immediate-early proteins BZLF1 and BRLF1, runs the full gene cascade, builds virions, kills the cell and sheds virus into the saliva.

The other options reverse these features.

Influenza A (an orthomyxovirus, class V) has a segmented negative-sense RNA genome and packages its RdRp. Rotavirus is also segmented and carries a polymerase, but its genome is double-stranded (class III), not negative-sense.

Fredricks and Relman (1996) recast the postulates as molecular guidelines (sequence detection, specificity, treatment response, temporality, plausibility, biological gradient, consistency). Rivers (1937) and Evans (1982) had earlier adapted the bacterial postulates to viruses.

Symmetry strictly describes the capsid or nucleocapsid; the envelope is not a rigid symmetric shell, so the whole particle varies in size and shape, even though the nucleocapsid inside is ordered.

Sixty is the maximum in strictly identical environments. Caspar and Klug showed that subunits in quasi-equivalent positions allow multiples of 60 (the triangulation number, T), building larger shells from a single protein.

Because the VZV glycoproteins are tagged with mannose-6-phosphate, newly enveloped virions are diverted into lysosomes and degraded in most tissues, so the virus spreads cell-to-cell rather than releasing free virus.

Maturing keratinocytes at the skin surface lose this receptor, allowing infectious cell-free virus into the vesicle fluid, the source of airborne transmission and of the virus that seeds the sensory nerves.

The light microscope resolves to about 300 nm; most viruses are smaller and so were invisible until electron microscopy. Poxviruses, the largest vertebrate viruses, sit right at that limit.

VZV carries a linear double-stranded DNA genome of about 125 kilobases, the smallest of the human herpesviruses. Unusually for an alphaherpesvirus it has no glycoprotein D ortholog, relying instead on glycoprotein E for cell-to-cell spread, and it is the only human herpesvirus against which licensed vaccines exist.

High priorityMCQ
A particle-to-infectivity ratio greater than 1000:1 for a virus stock means that:
- A. Most particles in the stock are infectious
- B. The plaque assay is overcounting infectivity
- C. Viral load exactly equals the infectious titre here
- D. The stock is contaminated
- E. Physical particles far outnumber those that form plaques
Show answer
Correct answer: E
Most particles never form a plaque. This is also why viral load (genome copies measured by PCR) is not the same as infectious titre: PCR cannot distinguish a whole virion from an empty shell.
High priorityMCQ
A positive-sense single-stranded RNA genome (such as a picornavirus) is infectious as naked RNA because:
- A. It carries a polymerase in the virion
- B. It acts directly as messenger RNA
- C. It is double-stranded
- D. It integrates into the host genome first
- E. It must be reverse-transcribed first
Show answer
Correct answer: B
Positive-sense RNA has the same sequence as mRNA, so a ribosome translates it immediately, producing the polymerase the virus then uses to replicate.
High priorityMCQ
A positive-sense single-stranded RNA genome is defined by the fact that it:
- A. Can act directly as messenger RNA
- B. Is complementary to mRNA and must be transcribed before translation
- C. Is always segmented
- D. Requires reverse transcriptase
- E. Is double-stranded at the 5' end
Show answer
Correct answer: A
Positive-sense RNA has the same sequence as mRNA and is translated directly (picornaviruses, flaviviruses, coronaviruses). Negative-sense RNA must first be transcribed by the virion’s own RNA-dependent RNA polymerase (RdRp).
High priorityMCQ
All RNA viruses must carry their own RNA-dependent RNA polymerase, because host cells lack one. Which viruses are the exception, replicating through a reverse transcriptase instead?
- A. Coronaviruses
- B. Paramyxoviruses
- C. Flaviviruses
- D. Retroviruses
- E. Orthomyxoviruses
Show answer
Correct answer: D
Retroviruses replicate through a DNA intermediate using reverse transcriptase (an RNA-dependent DNA polymerase) rather than an RdRp. The hepadnaviruses (hepatitis B) use the same enzyme, though they are DNA viruses.
High priorityMCQ
Cleavage of the influenza haemagglutinin precursor (HA0) into HA1 and HA2 by a host protease:
- A. Inactivates the protein completely
- B. Immediately triggers membrane fusion
- C. Adds the lipid envelope to the mature particle
- D. Is required for viral genome replication
- E. Leaves it metastable, primed but waiting to fuse
Show answer
Correct answer: E
Priming cleavage does not switch fusion on; it leaves the fusion protein metastable (“primed but waiting”). The trigger (low endosomal pH for influenza, or receptor binding for others) then releases it into its fusion-active form.
High priorityMCQ
Covalently closed circular DNA (cccDNA) is a hallmark of which virus?
- A. Adenovirus
- B. Poliovirus
- C. Hepatitis B virus
- D. Influenza A virus
- E. Rotavirus
Show answer
Correct answer: C
The hepadnavirus (HBV) genome is a partially double-stranded circle repaired to cccDNA in the nucleus; this persistent template is a major reason HBV is so hard to cure.
High priorityMCQ
Dolutegravir, the third drug of the standard South African first-line regimen, inhibits which enzyme?
- A. Reverse transcriptase
- B. Protease
- C. Integrase
- D. RNA-dependent RNA polymerase
- E. Neuraminidase
Show answer
Correct answer: C
Dolutegravir (and the long-acting cabotegravir) are integrase strand-transfer inhibitors that block insertion of the reverse-transcribed HIV DNA into the host genome.
High priorityMCQ
How does a virus's taxonomic class relate to its Baltimore class?
- A. They are synonymous
- B. They are unrelated concepts that share a word
- C. A Baltimore class is a rank within the taxonomic hierarchy
- D. The taxonomic class replaced the Baltimore system in 2018
- E. The taxonomic class sits just below the order rank
Show answer
Correct answer: B
The two are unrelated concepts that happen to share a word. The taxonomic class is one rung of the 15-rank hierarchy (suffix -viricetes); a Baltimore class groups viruses by the route from genome to messenger RNA.
High priorityMCQ
Non-enveloped viruses are classically released by cell lysis, whereas enveloped viruses:
- A. also lyse the cell to escape
- B. exit only by exocytosis of intact vesicles
- C. integrate into the host genome
- D. bud through a host membrane
- E. are released by neuraminidase alone
Show answer
Correct answer: D
Naked viruses generally accumulate and are freed when the cell bursts; enveloped viruses wrap themselves in host membrane as they bud out.
High priorityMCQ
Oseltamivir and zanamivir stop influenza spread by inhibiting neuraminidase, an enzyme that normally:
- A. Binds the host-cell sialic acid receptor
- B. Cleaves sialic acid to free progeny
- C. Copies the genome
- D. Cleaves the polyprotein
- E. Forms the M2 ion channel
Show answer
Correct answer: B
Haemagglutinin tethers budding virions to surface sialic acid; neuraminidase cleaves it to free them. Blocking neuraminidase traps progeny at the surface and halts spread.
High priorityMCQ
Regarding "measles virus" and *Morbillivirus hominis*, which is correct?
- A. They are interchangeable synonyms, both governed directly by the ICTV
- B. "Measles virus" is the formal species name, and the binomial is vernacular
- C. Both "measles virus" and Morbillivirus hominis are vernacular names
- D. Morbillivirus hominis is an obsolete name that is no longer used at all
- E. "Measles virus" is the vernacular agent name; Morbillivirus hominis the formal species
Show answer
Correct answer: E
The vernacular name denotes the agent; the binomial denotes the taxonomic species. Vernacular virus names are unregulated by the ICTV.
High priorityMCQ
RNA viruses evolve far faster than DNA viruses chiefly because:
- A. They have much larger genomes
- B. They recombine far more readily than DNA viruses
- C. Their polymerases lack proofreading
- D. They all replicate in the host nucleus
- E. They all have segmented genomes
Show answer
Correct answer: C
The RNA polymerases of RNA viruses do not proofread and make errors roughly ten thousand times more often than DNA polymerases, generating the quasispecies swarm that escapes immunity and drugs.
High priorityMCQ
Tenofovir and lamivudine, the nucleos(t)ide backbone of South African first-line ART, act by:
- A. Blocking integrase
- B. Inhibiting the viral protease
- C. Blocking the CCR5 co-receptor used for entry
- D. Preventing budding
- E. Chain-terminating the DNA made by reverse transcriptase
Show answer
Correct answer: E
They are chain-terminating nucleos(t)ide reverse-transcriptase inhibitors, active against HIV reverse transcriptase (and the HBV polymerase). Nevirapine, by contrast, is a non-nucleoside RT inhibitor that binds the enzyme allosterically.
High priorityMCQ
The abrupt 'antigenic shift' of influenza is caused by:
- A. Gradual point mutation, otherwise known as drift
- B. Reassortment of whole genome segments
- C. Recombination of DNA
- D. Proviral integration
- E. Error catastrophe
Show answer
Correct answer: B
Reassortment of segments between two influenza viruses in a single cell produces an abrupt, large antigenic change (shift), distinct from the gradual point-mutation drift.
High priorityMCQ
The Baltimore classification sorts viruses on what basis?
- A. Capsid symmetry (icosahedral, helical, complex)
- B. Presence or absence of a lipid envelope
- C. Host kingdom (animal, plant, fungal, bacterial)
- D. Whether the virus is oncogenic
- E. The route from genome to messenger RNA
Show answer
Correct answer: E
Baltimore sorts viruses into seven classes by the route from genome to messenger RNA. It is non-hierarchical and complements, rather than replaces, ICTV taxonomy.
High priorityMCQ
The expanded hierarchy added four new principal ranks. Which set is correct?
- A. Domain, kingdom, phylum, class
- B. Realm, kingdom, phylum, class
- C. Realm, division, phylum, cohort
- D. Empire, realm, kingdom, phylum
- E. Realm, kingdom, tribe, class
Show answer
Correct answer: B
Realm, kingdom, phylum and class were added, all sitting above order. The basal rank is “realm”, not “domain”.
High priorityMCQ
The HIV '-navir' drugs, the hepatitis C '-previr' drugs, and nirmatrelvir all work by:
- A. Preventing cleavage of the polyprotein
- B. Blocking reverse transcriptase
- C. Blocking viral attachment
- D. Inhibiting the host cell ribosome directly
- E. Degrading the viral genome
Show answer
Correct answer: A
These are protease inhibitors. A virus that translates its genome as one polyprotein must cut it into functional units with a viral protease; blocking that protease leaves the polyprotein uncut and the virions immature.
High priorityMCQ
The RNA-dependent RNA polymerase (RdRp) of RNA viruses is a particularly clean antiviral target because:
- A. It is identical to a host enzyme
- B. It is found only in DNA viruses, not RNA ones
- C. It proofreads efficiently
- D. Human cells have no equivalent enzyme, so inhibitors are selective
- E. It is non-essential for replication
Show answer
Correct answer: D
Because no host enzyme copies RNA from RNA, an RdRp inhibitor can be highly selective for the virus. Sofosbuvir (hepatitis C) and remdesivir (SARS-CoV-2) act here.
High priorityMCQ
The tissue tropism of a virus is determined chiefly by:
- A. Distribution of the host receptor it binds
- B. Its genome size
- C. Its Baltimore class
- D. The route by which it exits the cell
- E. Whether it is enveloped
Show answer
Correct answer: A
A virus can only infect cells that display the receptor its ligand binds, so receptor distribution largely dictates which tissues are infected and what disease results.
High priorityMCQ
The traditional five-rank hierarchy (species, genus, subfamily, family, order) was expanded by the ICTV into how many ranks, and over what period was the change ratified?
- A. 10 ranks, ratified 2012
- B. 12 ranks, ratified 2016–2017
- C. 15 ranks, ratified 2018–2019
- D. 15 ranks, ratified 2024–2025
- E. 8 ranks, ratified 2020
Show answer
Correct answer: C
The 15-rank, Linnaean-like hierarchy was ratified in two ICTV votes, in 2018 and 2019.
High priorityMCQ
Treating a virus with a lipid solvent such as ether or chloroform abolishes its infectivity. This indicates that the virus:
- A. Has a double-stranded DNA genome
- B. Is enveloped
- C. Is icosahedral
- D. Is a prion
- E. Replicates in the cytoplasm
Show answer
Correct answer: B
Lipid solvents and detergents dissolve the envelope and destroy the infectivity of enveloped viruses while leaving naked viruses intact. This is the basis of the ether-sensitivity test for an envelope.
High priorityMCQ
Viral capsids display only two true symmetries. They are:
- A. Cubic and spherical
- B. Icosahedral and helical
- C. Helical and pleomorphic
- D. Icosahedral and complex
- E. Tetrahedral and helical
Show answer
Correct answer: B
Only icosahedral and helical symmetry occur. Particles that are neither (poxviruses, the HIV-1 cone, tailed bacteriophages) are termed “complex”.
High priorityMCQ
Viral genomes are haploid, with one important exception. Which viruses carry a diploid genome?
- A. Herpesviruses
- B. Reoviruses
- C. Retroviruses
- D. Poxviruses
- E. Orthomyxoviruses
Show answer
Correct answer: C
Retroviruses are uniquely diploid, packaging two copies of their positive-sense ssRNA genome; every other viral genome is haploid.
High priorityMCQ
Viral taxonomy is "polythetic". What does this mean?
- A. Every member of a taxon must share one single essential defining property
- B. A taxon rests on shared properties, none individually essential
- C. A single virus may belong to several different taxa at once
- D. Taxa are defined purely by measured phylogenetic distance
- E. Membership is settled by majority vote of the Study Group members
Show answer
Correct answer: B
A polythetic class is defined by multiple shared characters, none individually required, which is why species rest on multiple criteria and tolerate within-species variation.
High priorityMCQ
Virus taxonomy is described as "deliberately non-systematic" and polyphyletic. Which statement best explains why?
- A. They mutate too fast for any classification to remain stable
- B. They cannot be sequenced reliably, so relationships are only guessed
- C. The ICTV randomises assignments deliberately to avoid systematic bias
- D. They arose from several independent origins, so fit no single tree
- E. They are not alive, so evolutionary terms cannot apply to them
Show answer
Correct answer: D
Unlike cellular life, viruses do not descend from a single common ancestor; the system reflects several separate roots, so there is no single virosphere-wide tree.
High priorityMCQ
What format does the ICTV now mandate for **every** virus species name?
- A. A single Latinised word ending in the suffix -virus, capitalised
- B. The vernacular virus name, written entirely in capitals
- C. A polynomial describing the genome type and natural host
- D. A binomial of a genus name plus a one-word epithet
- E. A unique numeric accession code issued on registration
Show answer
Correct answer: D
Species names are binomial: a genus name plus a single-word epithet, mirroring the Linnaean convention used across biology.
High priorityMCQ
When assigning a newly discovered virus, which rank(s) must a virologist assign as a minimum?
- A. Realm and species
- B. Family and genus
- C. Genus and species
- D. All 15 ranks
- E. Species only
Show answer
Correct answer: C
Only genus and species assignment is obligatory; higher ranks are populated only where the evidence justifies them, so most viruses are not placed at all 15 levels.
High priorityMCQ
Which best states the current ICTV concept of a virus *species*?
- A. Any group of viruses that happens to infect the same host
- B. A group of viruses sharing essentially identical genome sequences
- C. A monophyletic group distinguishable from others by multiple criteria
- D. A single virus isolate maintained as a reference strain
- E. Any cluster of viruses sharing one essential defining property
Show answer
Correct answer: C
The ICTV defines a species as a monophyletic group distinguished by multiple criteria, set per family by the relevant Study Group.
High priorityMCQ
Which feature is **essential** for a virus to be classed as an arbovirus?
- A. It replicates within a blood-feeding arthropod before transmission to a vertebrate
- B. It causes a haemorrhagic fever in the infected vertebrate host
- C. It is transmitted specifically by mosquitoes and no other arthropod vectors
- D. It is carried mechanically on arthropod mouthparts between hosts
- E. It belongs to the family Flaviviridae or a closely related family
Show answer
Correct answer: A
“Arbovirus” requires genuine replication in the arthropod vector; passive mechanical carriage (the “flying pin”) does not qualify. Arboviruses span several families.
High priorityMCQ
Which is a correctly formatted **current** (binomial) virus species name?
- A. Measles virus
- B. Measles morbillivirus
- C. MEASLES VIRUS
- D. Human morbillivirus 1
- E. Morbillivirus hominis
Show answer
Correct answer: E
The measles species is Morbillivirus hominis (genus plus one-word epithet, italicised, genus capitalised). “Measles virus” is the vernacular agent name; “Measles morbillivirus” is a superseded transitional form.
High priorityMCQ
Which of the following viruses has a helical nucleocapsid?
- A. Adenovirus
- B. Human papillomavirus
- C. Rabies virus
- D. Poliovirus
- E. Hepatitis B virus
Show answer
Correct answer: C
Rabies virus (a rhabdovirus) has a helical, enveloped nucleocapsid; the others are icosahedral. In vertebrate viruses, helical nucleocapsids are always enclosed within an envelope.
High priorityMCQ
Which rank–suffix pairing is correct?
- A. Order = -viridae
- B. Family = -virales
- C. Genus = -virinae
- D. Class = -viricota
- E. Realm = -viria
Show answer
Correct answer: E
Realm = -viria. (Order = -virales, family = -viridae, subfamily = -virinae, genus = -virus, phylum = -viricota, class = -viricetes.)
High priorityMCQ
Which set lists the three *primary* modern criteria used to delineate the main viral taxa?
- A. Genome type/character/sequence, replication strategy, virion structure
- B. Virion size, virion stability, antigenicity
- C. Host range, disease caused, geographic origin
- D. Nucleic acid type, serology, electron-microscopic morphology
- E. Baltimore class, transmission route, vaccine availability
Show answer
Correct answer: A
The three primary criteria are the genome (type, character, sequence), the replication strategy, and the virion structure. Options naming virion size or serology are the older, pre-molecular criteria.
High priorityMCQ
Which statement about the viral envelope is correct?
- A. Both its lipids and its proteins are virus-encoded
- B. Its lipids are host-derived, its embedded proteins virus-encoded
- C. It is a rigid protein shell with icosahedral symmetry
- D. It is synthesised de novo from viral lipid in the cytoplasm
- E. It makes the virus more resistant to lipid solvents
Show answer
Correct answer: B
The envelope is taken from a host membrane during budding, so its lipids are host-derived and vary with the budding site, while the glycoprotein spikes embedded in it are virus-coded.
High priorityMCQ
Which viruses must package their own polymerase inside the virion and carry it into the next cell?
- A. Negative-sense RNA and double-stranded RNA viruses
- B. Positive-sense RNA and single-stranded DNA viruses
- C. Double-stranded DNA viruses only
- D. Single-stranded DNA viruses only
- E. Reverse-transcribing DNA viruses only
Show answer
Correct answer: A
No host enzyme can copy RNA from an RNA template or translate a negative strand, so the negative-sense (class V) and double-stranded RNA (class III) viruses must bring their own polymerase. A positive-sense genome is translated directly on entry, so its polymerase is made on the spot.
High priorityMCQ
Why can a virus not simply present a polycistronic genome to be translated in a human cell?
- A. The genome is too large
- B. Host ribosomes reject all viral RNA
- C. The message must first be converted to double-stranded form
- D. Polycistronic RNA cannot be capped
- E. A ribosome reads only the first frame, then stops
Show answer
Correct answer: E
Eukaryotic ribosomes do not reinitiate at downstream genes, so viruses reach their other proteins by polyprotein cleavage, splicing, segmentation, ribosomal frameshifting, or an internal ribosome entry site (IRES).
High priorityMCQ
Why did chloroquine block SARS-CoV-2 in cultured kidney cells yet fail in patients?
- A. The dose achievable in patients was too low
- B. The virus is not pH-sensitive
- C. It blocks neuraminidase, an enzyme the virus does not have
- D. Airway cells enter by a surface route, not the endosome
- E. Resistance emerged rapidly
Show answer
Correct answer: D
Chloroquine raises endosomal pH and so blocks the cathepsin-L endosomal entry route used by the kidney cells; airway cells expressing TMPRSS2 cleave the spike at the plasma membrane and fuse there, never depending on the endosome.
MCQ
'Error catastrophe', exploited by mutagens such as ribavirin, occurs when:
- A. The virus runs out of nucleotide building blocks
- B. The genome becomes too small
- C. Proofreading is enhanced
- D. The mutation rate exceeds the error threshold
- E. The host clears the virus by antibody
Show answer
Correct answer: D
RNA genomes sit just below an error threshold that caps them near 30 kilobases; coronaviruses are the exception, with an nsp14 proofreading exonuclease. Pushing the rate past the threshold drives the population to extinction.
MCQ
'Quasi-enveloped' release refers to:
- A. Naked viruses leaving inside host membrane vesicles
- B. Enveloped viruses shedding their envelope before exit
- C. A defective interfering particle
- D. Budding through the nuclear membrane
- E. Lysis of enveloped viruses
Show answer
Correct answer: A
Poliovirus, coxsackievirus, hepatitis A and hepatitis E can exit cloaked in host vesicles; circulating hepatitis A is entirely quasi-enveloped, which shields it from antibody.
MCQ
Aciclovir is selective for herpes-infected cells because:
- A. A viral kinase must activate it first
- B. It can only enter infected cells
- C. It preferentially inhibits the host DNA polymerase
- D. It is an integrase inhibitor
- E. It is a protease inhibitor
Show answer
Correct answer: A
Aciclovir is monophosphorylated only by viral thymidine kinase (HSV/VZV); the activated drug then chain-terminates the viral DNA polymerase, so it is inert in uninfected cells. Ganciclovir works analogously for CMV, activated by the CMV UL97 kinase.
MCQ
Amantadine inhibits influenza A by:
- A. Blocking neuraminidase
- B. Inhibiting the viral polymerase that copies the genome
- C. Blocking the M2 channel that uncoating needs
- D. Preventing viral attachment to sialic acid receptors
- E. Inhibiting integrase
Show answer
Correct answer: C
M2 is a proton channel that acidifies the virion interior to allow uncoating; amantadine and rimantadine block it. Near-universal resistance has retired them clinically, but they remain the classic uncoating inhibitor.
MCQ
An internal ribosome entry site (IRES) lets a virus:
- A. Integrate its genome directly into the host cell chromosome
- B. Start translation inside a message, not at its start
- C. Carry its own ribosome
- D. Reverse-transcribe its genome
- E. Evade neutralising antibody
Show answer
Correct answer: B
An IRES is a structured RNA element that recruits the ribosome internally, one of several ways viruses bypass the rule that a eukaryotic ribosome reads only the first gene of a message.
MCQ
Diploid cell strains such as WI-38 and MRC-5 stop dividing after roughly fifty divisions because of:
- A. Viral cytopathic effect
- B. Spontaneous transformation
- C. Bacterial contamination
- D. Telomere shortening (the Hayflick limit)
- E. Gradual loss of their surface receptors
Show answer
Correct answer: D
Diploid strains senesce at the Hayflick limit as telomeres shorten; continuous lines such as HeLa and BHK-21 are immortalised and divide indefinitely but have drifted from normal tissue.
MCQ
Double-stranded RNA genomes among vertebrate viruses are characteristically:
- A. Monopartite and circular
- B. Positive-sense and capped
- C. Segmented, as in the reoviruses
- D. Diploid, like the retroviruses
- E. Replicated by a virion reverse transcriptase
Show answer
Correct answer: C
The dsRNA viruses (Reoviridae, Birnaviridae, Picobirnaviridae) all carry segmented genomes; rotavirus, a reovirus, has 11 segments.
MCQ
Embryonated hens' eggs remain in routine use chiefly for:
- A. Routinely growing herpesviruses in the laboratory
- B. Producing influenza virus and influenza vaccine
- C. Performing plaque assays
- D. Measuring viral load
- E. Reverse transcription
Show answer
Correct answer: B
Eggs, used since the 1930s, remain the workhorse for influenza virus stocks and much influenza vaccine manufacture.
MCQ
Epstein-Barr virus glycoprotein gp42 engages which molecule as the coreceptor for entry into B cells?
- A. CD21 (complement receptor 2)
- B. An integrin of the avb6 type
- C. Sialylated surface glycans
- D. The CD4 molecule
- E. An HLA class II molecule
Show answer
Correct answer: E
On a B cell gp42 binds an HLA (human leukocyte antigen) class II molecule as coreceptor, after gp350/220 has attached to CD21, and the gH/gL heterodimer with gB then drives fusion.

This underlies the tropism switch: epithelial cells, which lack CD21, are entered without gp42 (using integrins and other proteins), so virus made in a B cell carries little gp42 and infects epithelium well, while virus made in epithelium carries gp42 and infects B cells well. CD21 is the attachment receptor, not the gp42 coreceptor.
MCQ
For many virus–disease associations, modern virology favours which framing?
- A. Infection is followed by disease in every case
- B. The virus is a sufficient cause in all cases
- C. Viral causation cannot be established at all
- D. The virus is best regarded as a "risk factor"
- E. Detecting viral nucleic acid confirms causation
Show answer
Correct answer: D
For many virus–disease links, “risk factor” describes the relationship better than an absolute “cause”: disease appears in only a fraction of the infected, and a syndrome may have several causes.
MCQ
Groupings such as "enteric", "respiratory" and "blood-borne" viruses are best described as:
- A. Formal orders recognised within the ICTV taxonomic hierarchy
- B. Baltimore classes defined by the route from genome to messenger RNA
- C. Informal categories based on transmission and tropism, cutting across families
- D. Exact synonyms for particular named viral genera
- E. Realms sitting at the base of the 15-rank hierarchy
Show answer
Correct answer: C
These are informal categories defined by how a virus spreads and where it replicates; one family can appear in several, and they cut across the formal taxonomy.
MCQ
Hepatitis B virus is unusual among DNA viruses because it:
- A. Its genome is RNA rather than DNA
- B. It is single-stranded throughout
- C. It copies an RNA intermediate by reverse transcriptase
- D. It carries an RNA-dependent RNA polymerase like other RNA viruses
- E. It cannot be transcribed by host enzymes
Show answer
Correct answer: C
HBV (class VII) is a DNA virus that copies a pregenomic RNA intermediate back into DNA by reverse transcriptase, which is why reverse-transcriptase inhibitors such as tenofovir treat it.
MCQ
Hepatitis delta virus is classified in which realm?
- A. Ribozyviria
- B. Riboviria
- C. Duplodnaviria
- D. Adnaviria
- E. It is unassigned to any realm
Show answer
Correct answer: A
Ribozyviria contains small circular RNA agents with self-cleaving ribozymes, including hepatitis delta virus.
MCQ
HIV protease inhibitors leave newly budded virions:
- A. Assembled but immature and non-infectious
- B. Unable to attach to any host cell
- C. Unable to bud from the cell
- D. Lacking an envelope
- E. Hypermutated
Show answer
Correct answer: A
The protease normally cleaves the Gag-Pol polyprotein during maturation; inhibiting it leaves the particle assembled but immature and non-infectious.
MCQ
How does the Epstein-Barr virus protein LMP2A help the virus persist in B cells?
- A. By mimicking CD40 signalling
- B. By encoding a viral DNA polymerase
- C. By acting as a viral bcl-2 homologue
- D. By mimicking B-cell-receptor signals
- E. By tethering the episome to chromatin
Show answer
Correct answer: D
LMP2A delivers a tonic signal resembling that of the B-cell receptor, keeping the latently infected B cell alive and blocking the differentiation that would otherwise trigger reactivation, so the cell survives without antigen.

The CD40 mimic is LMP1; the bcl-2 homologue is the separate viral protein BHRF1; the episome is tethered by Epstein-Barr nuclear antigen 1 (EBNA1); a viral DNA polymerase belongs to the lytic cycle.
MCQ
How is Epstein-Barr virus classified?
- A. Alphaherpesvirus, genus Simplexvirus
- B. Betaherpesvirus, genus Cytomegalovirus
- C. Gammaherpesvirus, genus Lymphocryptovirus
- D. Gammaherpesvirus, genus Rhadinovirus
- E. Alphaherpesvirus, genus Varicellovirus
Show answer
Correct answer: C
Epstein-Barr virus is a gammaherpesvirus and the prototype of the genus Lymphocryptovirus; its current species name is Lymphocryptovirus humangamma4.

The other human gammaherpesvirus, Kaposi sarcoma-associated herpesvirus, sits in the genus Rhadinovirus. The alphaherpesviruses (herpes simplex, varicella-zoster) and betaherpesviruses (cytomegalovirus, human herpesviruses 6 and 7) are different subfamilies.
MCQ
How is the Epstein-Barr virus genome maintained in a latently infected cell?
- A. Integrated into a host chromosome
- B. As a circular episome tethered by EBNA1
- C. As linear DNA free in the cytoplasm
- D. As an RNA provirus made by reverse transcriptase
- E. Copied continuously by a viral DNA polymerase
Show answer
Correct answer: B
The linear double-stranded DNA genome (about 172 kilobases) circularises on entering the cell into a chromatin-coated episome that the viral protein Epstein-Barr nuclear antigen 1 (EBNA1) tethers to the host chromosomes; the host DNA polymerase then replicates it with the cell. It is not integrated.

Integration, a cytoplasmic linear genome, and an RNA provirus made by reverse transcriptase all describe other viruses; a viral DNA polymerase operates in the lytic cycle, not in latency.
MCQ
Human influenza viruses preferentially bind which form of sialic acid?
- A. The alpha-2,3-linked form (avian and deep-lung type)
- B. Heparan sulfate
- C. They do not bind sialic acid
- D. The CD4 receptor
- E. The alpha-2,6-linked form (human upper-airway type)
Show answer
Correct answer: E
Human strains prefer alpha-2,6-linked sialic acid (upper airway); avian strains prefer alpha-2,3 (avian gut and deep human lung). A shift in this preference is one of the features watched in pandemic surveillance.
MCQ
In a binomial species name, the genus component ends in which suffix?
- A. -viridae
- B. -virinae
- C. -virus
- D. -virales
- E. -viria
Show answer
Correct answer: C
The genus (and subgenus) suffix is -virus, as in Morbillivirus within Morbillivirus hominis.
MCQ
In the pre-molecular era, five properties were given roughly equal weight in classification. Which of the following was **not** one of them?
- A. Type of nucleic acid (DNA or RNA)
- B. Virion size
- C. Virion morphology
- D. Complete genome nucleotide sequence
- E. Virion antigenicity
Show answer
Correct answer: D
Whole-genome sequencing is a modern criterion, not one of the historical five (nucleic-acid type, virion size, morphology, stability, antigenicity).
MCQ
In virus attachment, which use of terms is correct?
- A. Receptor and ligand both refer to viral proteins
- B. The receptor is on the virus; the ligand on the host cell
- C. The ligand is the lipid envelope itself
- D. The receptor is on the host cell; the ligand on the virus
- E. The receptor is the viral glycoprotein spike
Show answer
Correct answer: D
The receptor is the host-cell molecule; the ligand is the viral molecule that binds it. For example, the SARS-CoV-2 spike is the ligand and ACE2 the receptor.
MCQ
Into which realm are HIV-1 and the other reverse-transcribing viruses placed?
- A. They have their own realm, Retroviria
- B. Varidnaviria
- C. Riboviria
- D. Duplodnaviria
- E. They are unassigned, being neither pure DNA nor pure RNA viruses
Show answer
Correct answer: C
Riboviria gathers RNA viruses replicating via an RNA-directed polymerase together with the reverse-transcribing viruses; HIV-1 sits in Riboviria, kingdom Pararnavirae.
MCQ
Labels such as serotype, genotype, subtype, variant and vaccine strain describe lineages **below** the species level. Who governs their naming?
- A. The ICTV, under the same rules as species
- B. The WHO exclusively
- C. GenBank, automatically on submission
- D. No one; such labels are not permitted
- E. Specialty groups, outside the ICTV's remit
Show answer
Correct answer: E
Sub-species labels are clinically and epidemiologically important but fall outside the ICTV’s remit; conventions vary from virus to virus.
MCQ
Lenacapavir, a long-acting HIV agent, targets the viral:
- A. Integrase
- B. Reverse transcriptase
- C. Capsid
- D. Protease
- E. gp41 fusion protein
Show answer
Correct answer: C
Lenacapavir binds the HIV capsid protein, disrupting both proper capsid assembly/maturation and the incoming capsid’s uncoating and nuclear import.
MCQ
Letermovir prevents CMV disease in transplant recipients by inhibiting the:
- A. CMV DNA polymerase (the ganciclovir target)
- B. Reverse transcriptase
- C. The viral maturation protease
- D. Neuraminidase
- E. CMV terminase that packages the genome
Show answer
Correct answer: E
Letermovir blocks the terminase complex that cuts concatemeric DNA and packages it into preformed capsids, an assembly/packaging step distinct from the DNA polymerase that ganciclovir inhibits.
MCQ
Maraviroc prevents HIV entry by:
- A. Inhibiting the reverse transcriptase enzyme
- B. Binding CD4
- C. Blocking the CCR5 co-receptor
- D. Inhibiting integrase
- E. Blocking neuraminidase
Show answer
Correct answer: C
Maraviroc is a CCR5 antagonist: it occupies the co-receptor so gp120/gp41 cannot complete entry. People homozygous for the CCR5-Δ32 deletion are naturally resistant to CCR5-tropic HIV for the same reason.
MCQ
Most DNA viruses replicate their genomes in the nucleus, whereas most RNA viruses replicate in the:
- A. nucleus
- B. cytoplasm
- C. mitochondrion
- D. endosome
- E. extracellular space
Show answer
Correct answer: B
Most DNA viruses replicate in the nucleus (poxviruses are the cytoplasmic exception); most RNA viruses replicate in the cytoplasm (influenza is a nuclear exception).
MCQ
On an icosahedral capsid such as adenovirus, the capsomeres at the twelve vertices, each bonded to five neighbours, are called:
- A. Hexons
- B. Peplomers
- C. Protomers
- D. Matrix units
- E. Pentons
Show answer
Correct answer: E
Vertex capsomeres bonded to five neighbours are pentons (pentamers); those on faces and edges, bonded to six, are hexons. An icosahedron has twelve vertices, hence twelve pentons.
MCQ
Paramyxoviruses and HIV differ from influenza in that they can penetrate by:
- A. Fusing at the plasma membrane at neutral pH
- B. Forming a pore in the membrane without any fusion
- C. Lysing the plasma membrane
- D. Reverse transcription at the surface
- E. Using neuraminidase to enter
Show answer
Correct answer: A
Some enveloped viruses fuse directly at the cell surface at neutral pH; others such as influenza require the low pH of the endosome to trigger fusion.
MCQ
Since 2018, what structural rule applies to every virus species?
- A. It must be assigned to a genus
- B. It must have a designated type species
- C. It must be assigned to all 15 ranks
- D. It must have a sequenced reference genome
- E. It must be named after its host
Show answer
Correct answer: A
Since 2018 every species must belong to a genus, a requirement not previously enforced. The type-species concept, by contrast, was abolished in 2021.
MCQ
The 'eclipse period' of the one-step growth curve is the interval during which:
- A. The virus is at its most infectious inside the cell
- B. Progeny are being released
- C. No infectious virus can be recovered from the cells
- D. The host cell divides
- E. Antibody first appears
Show answer
Correct answer: C
After uncoating and before the first progeny are assembled, no infectious particle exists, so none can be recovered even by breaking the cells open.
MCQ
The "hepatitis viruses" (A, B, C, D, E) are grouped together because they share the liver as their main target. Taxonomically, how related are they?
- A. They form a single family, Hepatitisviridae
- B. They belong to five entirely unrelated families
- C. They belong to two closely related genera
- D. They are all members of Riboviria
- E. They are all reverse-transcribing viruses
Show answer
Correct answer: B
Hepatitis A, B, C, D and E belong to five unrelated families; the grouping is by target organ, not taxonomy.
MCQ
The 2009 pandemic H1N1 influenza virus illustrates which evolutionary process, and from how many ancestral lineages?
- A. Antigenic drift, from a single lineage
- B. Reassortment, from four ancestral lineages
- C. Recombination, from two lineages
- D. De novo emergence, from no known ancestor
- E. Reverse transcription, from two lineages
Show answer
Correct answer: B
The 2009 H1N1 virus was a reassortant carrying segments from four ancestral lineages: North American swine, North American avian, human, and Eurasian swine influenza.
MCQ
The basal rank is called the "realm" rather than the "domain". Why?
- A. The term "domain" was already assigned to the prions
- B. It avoids implying the single common ancestry that "domain" carries
- C. The two words are synonymous, so the choice was arbitrary
- D. Realms are defined only by shared host range
- E. "Domain" is reserved for ranks above kingdom in cellular taxonomy
Show answer
Correct answer: B
A realm groups viruses by deeply shared genes and proteins, not descent from one ancestor, so it deliberately avoids the single common ancestry that “domain” implies for cellular life.
MCQ
The binomial species-name mandate was ratified and then effectively completed when?
- A. Ratified 2012, completed 2017
- B. Ratified 2018, completed 2019
- C. Ratified 2021, completed at the February 2025 vote
- D. Ratified 2024, completed 2024
- E. Still a draft proposal, not yet in force
Show answer
Correct answer: C
The binomial mandate was ratified in 2021 and essentially completed at the February 2025 ratification vote, so almost all species names are now binomial.
MCQ
The mature capsid of HIV-1 is best described as:
- A. Icosahedral, built on a T = 3 lattice
- B. Helical
- C. A rigid brick-shaped shell
- D. A fullerene cone of hexamers and pentamers
- E. Absent; HIV has no capsid
Show answer
Correct answer: D
The mature HIV-1 capsid is a fullerene cone: a lattice of CA hexamers closed by exactly twelve pentamers, curved into a cone. Neither icosahedral nor helical, so “complex”.
MCQ
The term 'nucleocapsid' refers to:
- A. The lipid envelope together with its surface glycoproteins
- B. The genome together with its surrounding capsid
- C. The matrix protein layer alone
- D. The capsid alone, without nucleic acid
- E. The complete enveloped virion
Show answer
Correct answer: B
Nucleocapsid = the capsid plus the nucleic acid it encloses. Where present, an envelope is added around it to form the complete virion.
MCQ
To which Baltimore class does HIV belong?
- A. I (double-stranded DNA)
- B. IV (positive-sense ssRNA, non-reverse-transcribing)
- C. V (negative-sense ssRNA)
- D. VI (positive-sense ssRNA, reverse-transcribing)
- E. VII (double-stranded DNA, reverse-transcribing)
Show answer
Correct answer: D
HIV is a retrovirus, class VI: a positive-sense ssRNA genome reverse-transcribed to DNA and integrated. Hepatitis B is class VII (dsDNA, reverse-transcribing).
MCQ
Viruses replicate inside membranous 'replication organelles' or factories partly in order to:
- A. Increase their genome size
- B. Allow the ribosome to translate polycistronic mRNA
- C. Acquire a lipid envelope
- D. Hide double-stranded RNA from innate sensors
- E. Avoid needing a polymerase
Show answer
Correct answer: D
Sequestering replication concentrates the enzymes and conceals double-stranded RNA, the danger signal that RIG-I and MDA-5 detect to trigger interferon.
MCQ
What is now frequently the *earliest* step in placing a newly encountered isolate into a taxon?
- A. Inoculation into experimental animals
- B. Genome sequencing compared against reference databases
- C. Negative-contrast electron microscopy
- D. Serological neutralisation testing
- E. Virion-stability testing across pH and temperature
Show answer
Correct answer: B
A sequence compared against reference genomes places an isolate in a taxon almost immediately, so partial sequencing is often the earliest step.
MCQ
Where does cytomegalovirus (CMV) principally establish lifelong latency?
- A. Sensory dorsal root ganglion neurons
- B. Resting memory B lymphocytes
- C. Renal tubular epithelial cells
- D. Hepatocytes and Kupffer cells
- E. Monocytes and CD34 myeloid progenitors
Show answer
Correct answer: E
Why E

As a betaherpesvirus, CMV is latent in the myeloid lineage: in CD34 bone-marrow progenitors and the monocytes derived from them. Reactivation accompanies differentiation into macrophages and dendritic cells, which is why it reawakens during inflammation and immunosuppression.

For contrast, the alphaherpesviruses (herpes simplex, varicella-zoster) are latent in sensory ganglion neurons, and Epstein–Barr virus is latent in memory B lymphocytes.
MCQ
Where is the matrix protein of an enveloped virus located, and what does it do?
- A. On the outer surface, where it mediates receptor binding
- B. Within the genome, priming replication
- C. Forming the icosahedral capsid shell
- D. It is a host protein with no viral function
- E. Lining the inner face of the envelope, giving rigidity
Show answer
Correct answer: E
The non-glycosylated matrix protein (for example influenza M1) lines the inner face of the envelope, conferring rigidity and bridging the envelope and the nucleocapsid.
MCQ
Which body is responsible for the single universal system of virus classification and the nomenclature of virus taxa?
- A. The International Committee on Taxonomy of Viruses (ICTV)
- B. The World Health Organization (WHO)
- C. The National Center for Biotechnology Information (NCBI)
- D. The International Union of Microbiological Societies, by direct member vote
- E. Each national reference laboratory, for its own region
Show answer
Correct answer: A
The ICTV is the single body that sets viral taxonomy and the naming of taxa. NCBI and GenBank host sequence data but do not classify viruses.
MCQ
Which cell-surface molecule is the receptor that Epstein-Barr virus uses to attach to B cells?
- A. CD4 (the T-helper marker)
- B. CD46 (a complement regulator)
- C. ICAM-1 (an adhesion molecule)
- D. CD21 (complement receptor 2)
- E. CD155 (the poliovirus receptor)
Show answer
Correct answer: D
Epstein-Barr virus attaches to B cells when gp350/220 binds CD21, the complement receptor 2 that normally recognises the C3d fragment of complement; gp42 then engages an HLA (human leukocyte antigen) class II molecule as coreceptor.

The other options are receptors used by unrelated viruses: CD4 by HIV, CD46 by measles, ICAM-1 by major-group rhinoviruses, and CD155 by poliovirus.
MCQ
Which Epstein-Barr virus glycoprotein mediates attachment to B cells?
- A. gp350
- B. gp42
- C. gB
- D. gH/gL
- E. LMP1
Show answer
Correct answer: A
The major envelope glycoprotein gp350/220 attaches the virus to the B-cell surface by binding complement receptor 2 (CD21).

gp42 then acts as the coreceptor-binding protein, and gB with the gH/gL heterodimer drives membrane fusion. LMP1 is a latent membrane oncoprotein, not a virion attachment protein.
MCQ
Which family is **not** generally listed among the viruses with oncogenic potential?
- A. Papillomaviridae
- B. Hepadnaviridae
- C. Herpesviridae
- D. Retroviridae
- E. Picornaviridae
Show answer
Correct answer: E
Oncogenic potential is recognised in Herpesviridae, Adenoviridae, Papillomaviridae, Polyomaviridae, Hepadnaviridae, Retroviridae and Flaviviridae, but not the Picornaviridae.
MCQ
Which host machinery do most enveloped viruses hijack to pinch off a budding particle?
- A. The proteasome
- B. The host ribosome
- C. The Golgi apparatus acting on its own
- D. Neuraminidase
- E. The ESCRT membrane-scission complex
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Correct answer: E
Viral “late-domain” motifs recruit the host ESCRT machinery to perform the final membrane scission. Influenza is the exception, severing itself with its M2 protein.
MCQ
Which realm is defined by the HK97-fold major capsid protein?
- A. Riboviria
- B. Ribozyviria
- C. Varidnaviria
- D. Duplodnaviria
- E. Adnaviria
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Correct answer: D
Duplodnaviria gathers double-stranded DNA viruses sharing the HK97-fold capsid (herpesviruses, tailed bacteriophages). Varidnaviria is defined instead by the jelly-roll fold.
MCQ
Which statement about formal versus vernacular usage is correct?
- A. Formal taxon names are lowercase, with the rank label written after the name
- B. Vernacular names are italicised and capitalised like formal taxon names
- C. Vernacular names are assigned by the ICTV Study Groups themselves
- D. Formal names are capitalised and italicised; vernacular names are lowercase and unregulated
- E. There is no practical difference between the two in usage
Show answer
Correct answer: D
Formal taxon names are capitalised and italicised with the rank label preceding (the genus Morbillivirus); vernacular names are lowercase, not italicised, and set informally outside ICTV control.
MCQ
Which statement about virus stability is correct?
- A. Naked enteric viruses (rotavirus, enteroviruses) survive stomach acid
- B. Enveloped viruses are generally more heat-stable than naked enteric viruses
- C. All viruses are inactivated within seconds at 4 °C
- D. Prions are readily destroyed by boiling
- E. Freezing at −70 °C rapidly inactivates most viruses
Show answer
Correct answer: A
Naked enteric viruses (rotavirus, enteroviruses) resist stomach acid, fitting their faecal-oral spread, whereas enveloped viruses are inactivated at pH 5–6. Enveloped viruses are also the more heat-labile, −70 °C preserves infectivity for years, and prions resist boiling.
MCQ
Which statement best describes VZV latency?
- A. It occurs in circulating B lymphocytes
- B. It is maintained by continuous full lytic gene expression
- C. It is restricted to thoracic skin cells
- D. It is established in neurons, including enteric ganglia
- E. It never involves the cranial or autonomic nerve ganglia
Show answer
Correct answer: D
VZV establishes lifelong latency in neurons, and not only in the dorsal-root sensory ganglia but also in cranial-nerve, autonomic and enteric ganglia.

In the latent state the virus shuts down almost all gene expression, restricting itself largely to a non-coding latency-associated transcript that suppresses ORF61 and so blocks the cascade that would otherwise drive replication.
MCQ
Which statement best reflects the current role of cryo-electron microscopy (cryo-EM) in virus structure?
- A. It remains low-resolution, useful only for imaging whole infected cells
- B. It has been largely superseded by negative staining
- C. It can be used only on crystallised samples
- D. It routinely reaches near-atomic resolution and dominates virus-particle imaging
- E. It works only on viruses larger than 300 nm
Show answer
Correct answer: D
Cryo-EM now routinely achieves near-atomic resolution and has become the dominant technique for virus particles and their protein machines; X-ray crystallography is reserved for small viral enzymes and antibody-fragment complexes.
MCQ
Which statement correctly distinguishes Epstein-Barr virus latency from the lytic cycle?
- A. Latency produces infectious virus, the lytic cycle none
- B. Latency expresses few genes and makes no virus
- C. Lytic infection persists silently for life
- D. Latency is driven by the BZLF1 immediate-early protein
- E. The lytic cycle is the lifelong reservoir state
Show answer
Correct answer: B
In latency the genome persists as a quiet episome expressing only a restricted set of genes, with no virus produced; this is the state of the lifelong memory B-cell reservoir. The lytic cycle, triggered by the immediate-early proteins BZLF1 and BRLF1, runs the full gene cascade, builds virions, kills the cell and sheds virus into the saliva.

The other options reverse these features.
MCQ
Which virus has a segmented, negative-sense RNA genome and carries its own polymerase in the virion?
- A. Poliovirus
- B. Influenza A virus
- C. Hepatitis B virus
- D. Herpes simplex virus
- E. Coronavirus
Show answer
Correct answer: B
Influenza A (an orthomyxovirus, class V) has a segmented negative-sense RNA genome and packages its RdRp. Rotavirus is also segmented and carries a polymerase, but its genome is double-stranded (class III), not negative-sense.
MCQ
Who reframed the Henle–Koch postulates for the sequencing era (molecular detection)?
- A. Fredricks and Relman (1996)
- B. Thomas Rivers (1937)
- C. Alfred Evans (1982)
- D. David Baltimore (1971)
- E. Robert Koch, in the original bacterial formulation
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Correct answer: A
Fredricks and Relman (1996) recast the postulates as molecular guidelines (sequence detection, specificity, treatment response, temporality, plausibility, biological gradient, consistency). Rivers (1937) and Evans (1982) had earlier adapted the bacterial postulates to viruses.
MCQ
Why are many enveloped viruses, such as influenza and HIV, described as pleomorphic?
- A. The lipid envelope is not a rigid shell
- B. Their capsids switch between icosahedral and helical forms
- C. They have no internal order at all
- D. They change their genome from particle to particle
- E. They are defective, non-infectious particles
Show answer
Correct answer: A
Symmetry strictly describes the capsid or nucleocapsid; the envelope is not a rigid symmetric shell, so the whole particle varies in size and shape, even though the nucleocapsid inside is ordered.
MCQ
Why can an icosahedral capsid contain more than 60 identical subunits?
- A. The subunits are chemically different from one another
- B. The genome forces the extra subunits in
- C. The capsid switches to helical symmetry
- D. The extra subunits are host-derived
- E. Subunits sit in quasi-equivalent positions, flexing slightly
Show answer
Correct answer: E
Sixty is the maximum in strictly identical environments. Caspar and Klug showed that subunits in quasi-equivalent positions allow multiples of 60 (the triangulation number, T), building larger shells from a single protein.
MCQ
Why is infectious cell-free VZV released mainly from the superficial skin rather than from other infected tissues?
- A. Maturing keratinocytes lose the mannose-6-phosphate receptor
- B. Skin cells uniquely carry no viral glycoproteins
- C. The virus can replicate only in epidermal cells
- D. Antibody neutralises the virus in deeper tissues
- E. Surface keratinocytes overexpress the mannose-6-phosphate receptor
Show answer
Correct answer: A
Because the VZV glycoproteins are tagged with mannose-6-phosphate, newly enveloped virions are diverted into lysosomes and degraded in most tissues, so the virus spreads cell-to-cell rather than releasing free virus.

Maturing keratinocytes at the skin surface lose this receptor, allowing infectious cell-free virus into the vesicle fluid, the source of airborne transmission and of the virus that seeds the sensory nerves.
MCQ
Why was the development of electron microscopy essential to the study of viruses?
- A. Most viruses are smaller than the light microscope's ~300 nm resolution limit
- B. Viruses cannot be propagated in cell culture without electron microscopy
- C. Light microscopy cannot detect viral nucleic acid at all
- D. It is the only method that can determine a full genome sequence
- E. Viruses are transparent to visible light only when alive
Show answer
Correct answer: A
The light microscope resolves to about 300 nm; most viruses are smaller and so were invisible until electron microscopy. Poxviruses, the largest vertebrate viruses, sit right at that limit.
MCQ
Which statement about VZV virology is correct?
- A. It has the largest of the human herpesvirus genomes
- B. It has the smallest human herpesvirus genome
- C. It relies on glycoprotein D for cell entry
- D. No licensed vaccine exists against it
- E. It is a double-stranded RNA virus
Show answer
Correct answer: B
VZV carries a linear double-stranded DNA genome of about 125 kilobases, the smallest of the human herpesviruses. Unusually for an alphaherpesvirus it has no glycoprotein D ortholog, relying instead on glycoprotein E for cell-to-cell spread, and it is the only human herpesvirus against which licensed vaccines exist.