new: mike tutors and offers personalized advice for the usmle (click here for more info)

Thursday, May 3, 2007

Viruses are High Yield for the USMLE Step 1

I suggest when trying to memorize viral structures make a priority list. Common viruses and ones that also overlap with pathology are the ones to focus on here since they can be asked about in many different ways.

Knowing DNA vs RNA is the most important distinguishing feature. Furthermore, f you know if its DS or SS you'll probably get the points.

Enveloped vs Naked: If you can memorize enveloped vs. naked for each virus then god bless, but I like this:
Envelopes are made of a lipid bilayer which are prone to destruction by the environment whereas naked viruses are more resistant to environmental stresses.

A 46 year old female who has genital warts (d/t HPV) takes a shower in her home during which time the virus sheds on the floor. Later her child presents with a wart on his toe.
This all happened since this virus had its tough nucleocapsid for protection and not a wimpy envelope. Take home message is if the virus is capable of fomite transmission it is probably naked.

Play odds, especially for rare viruses with less of a chance of being asked. I walked into the exam knowing that if I saw an RNA virus that looked deadly or pretty rare I would guess that it is SS (-) linear w/envelope.

Here is my list of High Yield Viruses and why I think they have a great chance of being asked
  • Herpesviruses - DS linear DNA
    • this family includes so many viruses that overlap with pathology I list it first since I think this is most important.
  • All the viruses that cause hepatits - see First Aid for them since these are important!
    • Note that Hep B carries the reverse transcriptase enzyme with it just like the retroviruses do.
  • HIV - SS (+) linear RNA
    • If you don't get at least 1 question on AIDs I will give you my blog. Seeing if you know virus structure is a possible secondary question.
  • Papovavirus - DS (-) linear
    • besides HPV causing cervical cancer don't forget about molluscum contagiosum which causes the umbilicated wart (center of wart is depressed like an "innie" belly button).
  • Paramyxoviruses - SS (-) linear RNA with helical capsid symmetry
    • this family is responsible for causing croup (parainflunza virus), bronchiolitis (d/t RSV), measles and mumps.
  • Parvovirus - SS linear DNA
    • It, along with Hepadnavirus, is exception to the rule that all DNA viruses are linear.
    • It overlaps with pathology since it causes aplastic anemia in patients with sickle cell disease
      • Pt might give you a history of having recent contact with a kid who had a slapped cheek appearance.
      • Blood labs will give you a low RBC count with low reticulocytes (less than or equal to 3% of hematocrit - I wouldn't freak out about not knowing how to correct a reticulocyte count).
  • Influenza virus (an orthomyxovirus) - SS (-) linear segmented RNA
    • the NBME and practice questions I've seen love the fact that this genome is segmented since:
      • Genetic shift - if our influenza virus recombines with a pig's or chickens genome we get pandemics - we're all screwed (small minor mutations lead to drifts causing an epidemic in a much smaller area w/ much less people). involved)
      • If you digest the genome and view it with northern blotting you will see each of these (7 or 8) segments as separate bands.


Anonymous said...

Thank you mike

Kaplan Step 1 said...

I have purchased Kaplan Step 1 from and i am pretty much satisfied from their prices and service. On the top of all, after sale service is unprecedented. So if someone is looking to purchase used books for usmle exam preparation then I would suggest them to check this website.
Kaplan Step 1

Anonymous said...

I double-checked Papovaviruses, and you are correct they do cause HPV, however molluscum contagiousum is caused by Poxvirus (same as smallpox and vaccinia). Poxviruses are enveloped DNA viruses that replicate OUTSIDE the nucleus (only DNA viruses that do that, if I'm not mistaken)