The term 'isometric', applied to a virus particle, indicates that it
Answer: D
Isometric describes a particle whose dimensions are much the same in every direction, which is how an icosahedral capsid looks under the microscope. Helical nucleocapsids are elongated instead, and complex particles fall into neither description.
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Viruses are most commonly grown in the laboratory in
Answer: A
A virus needs living cells, so it cannot be grown on the sterile media that support bacteria however rich they are. Cell culture is the routine method, with animal inoculation kept for the minority of viruses that will not grow in it.
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Which of these is used when a virus cannot be isolated in cell culture?
Answer: B
Isolation occasionally requires inoculation into animals: suckling mice, rabbits, guinea pigs, hamsters or primates. The choice depends on which host supports that virus, and the practice persists for agents cell lines will not sustain.
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Which group of viruses is noted as not being isolable in cell culture systems?
Answer: C
Hepatitis viruses are singled out as agents that cannot be recovered in cell culture, which is why their diagnosis rests on serology and on detecting viral antigens or nucleic acid rather than on isolation. The other three groups grow readily in culture.
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An embryonated egg is useful for growing viruses because it
Answer: D
The developing embryo and its supporting membranes offer several different tissues at once, so a single system can support a wide variety of viruses that differ in which cells they can enter. That diversity is what makes the egg valuable rather than any chemical property.
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Plaques in a virus plaque assay are
Answer: A
A plaque forms where an infection has spread through neighbouring cells and killed them, leaving a clear patch in an otherwise intact monolayer. Viruses form no colonies, because they do not grow on media, only in cells.
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A plaque assay measures
Answer: B
Each plaque records that a virus was able to enter a cell, replicate, spread to its neighbours and damage them, so the count reflects infectivity rather than particle number. Many particles in a preparation are non-infectious and are invisible to this assay.
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Evidence of viral growth in cell culture is commonly seen as
Answer: C
Damage to the cell sheet, or inclusions visible within cells, signals that a virus is replicating. Turbidity, gas and fermentation reactions are all bacterial phenomena, since they depend on metabolism the virus does not possess.
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In serodiagnosis of a viral infection, the antibody class appearing relatively rapidly after infection is
Answer: D
IgM appears early, so finding it suggests a recent or current infection and can support a diagnosis from one sample. IgG rises later and persists, which is why it is followed across two samples rather than read on its own.
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A rising titre of IgG antibody is demonstrated by testing
Answer: A
Two samples taken at different points in the illness are compared, because a single IgG result cannot distinguish present infection from a past one. The comparison is what makes the rise, rather than the absolute value, the meaningful finding.
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