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-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
The AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.
This month’s stumper
A novel infectious disease has become prevalent and increasingly widespread over the past two years. The Centers for Disease Control has just isolated the cause of the illness, which is a virus. The virus looks like a brick-shaped complex, includes an envelope, and has a DNA genome. Replication of the viral DNA appears to occur in the cytoplasm via a virion-associated transcriptase.
This new virus would be most likely classified in which of the following families?
A. Adenovirus.
B. Hepadnavirus.
C. Herpesvirus.
D. Papillomavirus.
E. Parvovirus.
F. Poxvirus.
The correct answer is F.
Kaplan Medical explains why
Examples of viruses in the Poxvirus family are variola, vaccinia, and molluscum contagiosum. All DNA viruses are icosahedral, except for Poxviruses, which are described as brick-shaped complexes. Poxvirus have a DNA genome and are enveloped viruses. All DNA viruses replicate their DNA in the nucleus, except for Poxvirus, which replicates in the cytoplasm via a virion-associated transcriptase. All the enzymes and factors necessary for viral replication are then made in the cytoplasm.
Why the other answers are wrong
Choice A: Adenoviruses, like the other DNA viruses (except Poxvirus), are icosahedral and have double-stranded linear DNA that replicates in the nucleus. The DNA is not enveloped, and there is no virion-associated polymerase.
Choice B: The main example of the Hepadnavirus family is the hepatitis B virus. It is icosahedral, enveloped and contains partially double-stranded circular DNA that is enveloped. Viruses carry a DNA polymerase with reverse transcriptase activity to synthesize an RNA intermediate that is then used to make genomic DNA. This family of viruses replicates in the nucleus, differentiating it from the Poxvirus family.
Choice C: Herpesviruses consist of herpes simplex, varicella zoster, Epstein-Barr, and cytomegalovirus. These are icosahedral and contain enveloped, double-stranded, linear DNA. The DNA replicates in the nucleus and the virus is assembled in the nucleus, which differentiates it from the Poxvirus family.
Choice D: Papillomaviruses, most important of which is papillomavirus, are icosahedral and contain naked, double-stranded, circular DNA that replicates in the nucleus. All of these characteristics distinguish this virus family from the Poxvirus family.
Choice E: The most important of the Parvoviruses is parvovirus B19. It is icosahedral and is the only DNA virus that contains single-stranded DNA, which is naked and replicates in the nucleus.
Tips to remember
- Poxviruses are described as brick-shaped complexes.
- Poxvirus is an enveloped DNA virus. All DNA viruses replicate their DNA in the nucleus, except for Poxvirus, which replicates in the cytoplasm via a virion-associated transcriptase.
- All of the enzymes and factors necessary for viral replication are then made in the cytoplasm.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
