If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test takers. Check out this example from Kaplan Medical, and view an expert video explanation of the answer. Also check out all posts in this series.
This month’s stumper
An investigator is studying virus life cycles. He creates a continuous culture of kidney fibroblasts that are suitable hosts for a large variety of viral agents. In one experiment, the nuclei of these cells are removed by cytosurgery, and various viral agents are added to the cultures. Following culture of the viruses with the enucleated cells, the yield of cytopathic units of virus is quantified.
Which of the following viruses would be capable of replication in these enucleated cells?
A. Adenovirus
B. Cytomegalovirus
C. Influenza virus
D. JC virus
E. Poliovirus
The correct answer is E.
Kaplan Medical explains why
Most RNA viruses—for example, poliovirus—replicate in the cytoplasm and therefore can replicate in enucleated cells. Poliovirus belongs to the family Picornaviridae. These viruses are nonenveloped and have an icosahedral nucleocapsid that contains positive-sense RNA.
Why the other answers are wrong
Read these explanations to understand the important rationale for why each answer is incorrect.
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Choice A: Adenoviruses are non-enveloped and have an icosahedral nucleocapsid that contains a double-stranded linear DNA genome. All DNA viruses except the family Poxviridae carry out their replication in the nucleus, and therefore would not be capable of completing their life cycles in enucleated cells. Indeed, poxviruses are DNA viruses that replicate entirely in host cell cytoplasm.
Choice B: Cytomegalovirus is a member of family Herpesviridae. It is an enveloped virus with an icosahedral nucleocapsid that contains a double-stranded linear DNA genome, and thus replicates in the nucleus.
Choice C: The exception to the rule regarding RNA viruses is the family Orthomyxoviridae, the influenza viruses. Orthomyxoviruses undergo transcription and RNA replication in the nucleus of the host cell because they need to cannibalize the capped 5' termini of cellular RNAs for use as primers for viral mRNA transcription.
Choice D: JC virus belongs to family Polyomaviridae. It is nonenveloped and has an icosahedral nucleocapsid that contains a double-stranded circular DNA genome. It would not be able to complete its life cycle in an enucleated cell.
Tips to remember
- All RNA viruses replicate in cytoplasm, except Orthomyxoviruses and Retroviruses that have replicative stages in nuclei.
- All DNA viruses require a nucleus, except Poxviruses that can replicate in the cytoplasm.
For more prep questions on USMLE Steps 1 and 2, view other posts in this series.
