NBME 22 Answers : Block 4 : #9 = A 5-year-old boy is brought to the physician ...

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submitted by its_raining_jimbos(16),

Our little friend has a Parvovirus infection, which infects erythroid precursors, causing interruption of erythrocyte production. This is the same way it causes hydrops fetalis in unborn babies and aplastic anemia in sickle cell, etc.

gainsgutsglory I get Parvo has tropism for RBC precursors, but wouldn’t it take 120 days to manifest? +

keycompany RBCs don’t just spill out of the bone marrow every 4 months on the dot. Erythropoesis is a constant process. If you get a parvo virus on “Day 1” then the RBCs that were synthesized 120 days before “Day 1” will need to be replaced. They can’t be because of parvovirus. This leads to symptomatic anemia within 5 days because the RBCs that were synthesized 125-120 days before the infection are not being replaced. +7

drdoom @gainsgutsglory @keycompany It seems unlikely that “1 week” of illness can explain such a large drop in Hb. It seems more likely that parvo begins to destroy erythroid precursors LONG BEFORE it manifests clinically as “red cheeks, rash, fever,” etc. Might be overkill to do the math, but back-of-the-envelope: 7 days of 120 day lifespan -> represents ~6 percent of RBC mass. Seems unlikely that failure to replenish 6 percent of total RBC mass would result in the Hb drop observed. +

yotsubato He can drop from 11 to 10 hgb easily +1

ls3076 Apologies if this is completely left-field, but I didn't think this was Parvovirus. Parvo would affect face. Notably, patient has fever and THEN rash, which is more indicative of Roseola. Thoughts?? +3

hyperfukus @is2076 check my comment to @hello I thought the same thing for a sec too :) +

hyperfukus also i think you guys are thinking of hb in adults in this q it says hb is 10g/dL(N=11-15) so it's not relatively insanely low +

angelaq11 @Is3076 I completely agree with @hyperfukus and I think that thinking of Roseola isn't crazy, but remember that usually with Roseola you get from 3-5 days of high fever, THEN fever is completely gone accompanied by a rash. This question says that the patient has a history of 4 days of rash and 7 days of fever, but never mentioned that the fever subsided before the appearance of the rash. And Roseola is not supposed to present with anemia. +2

suckitnbme @Is3076 another point is that malar rash refers to the butterfly rash on the cheeks that is commonly seen in lupus, so the face is NOT spared. +