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NBME 23 Answers

nbme23/Block 3/Question#38

A 5-year-old boy who has recently recovered from an ...

Immune destruction of platelets

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 +6  upvote downvote
submitted by sajaqua1(186),

The marrow shows precursors to erythrocytes, as well as megakaryocytes. This is to show you that the issue isn't underproduction, which means that we are losing RBC and platelets somewhere ie destruction. That rules out D and E. There is nothing to indicate tha the child has disseminated tuberculosis (B). At this point we are left with A or C. A would indicate Disseminated Intravascular Coagulation (DIC) or something similar, which would result in low platelets and RBC but we would also see abnormal RBC like schistocytes ("helmet" cells). We are explicitly told that the erythrocytes are normochromic and normocytic. However, immune destruction of platelets explains it all- the destruction of platelets leads to some hemorrhaging and so a drop in RBC, and ITP classically arises after a recent upper respiratory tract viral infection.

meningitis  Just in case anyone is wondering like I did, the low platelet count explains thethose multinucleated cells. They are Megakaryocytes in Bone Marrow Biopsy. +1  
nwinkelmann  Also, don't forget that autoimmune thrombocytopenia purpura has 2 demographics: young kids, which generally resolves spontaneously fairly quickly, and then young adult females which is a true autoimmune condition that doesn't resolve. Patient's age + thrombocytopenia + essentially normal rest of heme pannel = autoimmune thrombocytopenia purprua in child. +1  




 +2  upvote downvote
submitted by seagull(392),

Why is this not HUS? How did you guys approach the question?

joonam  I think if this was HUS (d/t a bacterial infection) the leukocyte count would be abnormal (11k<) +  
yotsubato  normochromic normocytic RBC thats why. You would see schistocytes +1  




 +0  upvote downvote
submitted by consistentwrongdoer3(6),

What are we supposed to be looking at? I see multinucleated giant cells. I also see infiltrate (can’t tell if this is mononuclear or not).

methylased  Young child following URI with TCP is pretty classic ITP. Sometimes they throw in extra stuff on purpose, but I didn’t see much on the bone marrow aspirate either. +1  
mousie  I was also thinking ITP but the bone marrow image kind of threw me off too, not sure what I'm supposed to see but still think ITP is best choice ... +  
meningitis  It also almost threw me off, but then I remembered he had low platelet count and I guessed those multinucleated cells were Megakaryocytes (I looked for Megakaryocyte Bone Marrow Biopsy in google and they are the same). +1  
what  Bone marrow shows increased megakaryocytes -> ITP +