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NBME 23 Answers

nbme23/Block 1/Question#31

A 16-month-old boy is brought to the physician for ...

Absence of germinal centers in lymph nodes

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I’m going to say it’s X linked agammaglobulinemia rather than SCID, but the difference between these two are tiny but this is why I think it’s the former:

  • Boy (increased risk but both BA and SCID are x linked)
  • Recurrent bacterial infections but don’t mention diarrhea or thrush which is in SCID
  • Timeline is after 6 months, so the mother’s antibodies wore off.

SCID should be immediately because they just don’t have the IL2 receptors. CVID shows up when they’re 20-40 years old. You get absent germinal centers in both. No mention of absent thymic shadow which is in SCID.

placebo079  “Uniformly” low is also a clue; in CVID they are not. +  
tea-cats-biscuits  This makes sense, though what really threw me off was that in Classic Bruton’s Agammaglobulinemia there’s near-zero B counts though (or at least that’s what FA and UTD says, “Absent B cells in peripheral blood” FA 116, 2018). The Q says the leukocyte count was normal though. Wouldn’t the leukocyte count include lymphocytes in the differential? And wouldn’t lymphocytes be low due to the near complete lack of B cells in peripheral circulation if it was BA? +  
partybrockk  @tea-cats-biscuits Bruton’s is a failure of B cells to /MATURE/. So you get normal lymphocyte counts, decreased levels of immunoglobulins, and absent germinal centers. +1  
tea-cats-biscuits  @partybrockk That makes sense to me, but I keep getting hung up on how that’s not what either FA or UTD says about the classic lab findings of XLA. UTD specifically says this: “Laboratory findings include hypogammaglobulinemia/agammaglobulinemia, deficient antibody responses to immunizations, and absent or markedly reduced B cells in the blood,” and I previously quoted FA. I suppose it doesn’t really matter, but it’s definitely a bit frustrating unless I’m missing something about how absent B cells in the blood wouldn’t correlate to a decreased lymphocyte count ... +  
temmy  please correct me if i am wrong cos i might be but my logic was there is decreases immunoglobulin uniformly meaning the B cells are uniformly absent and since they develop in the germinal center, the germinal center will be absent. +  




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submitted by lilamk(5),

I made a lucky guess and chose this but I don’t think for the right reasons. I thought maybe he has BTK deficiency/Bruton’s Agammaglobinemia. But, now that I am going over it I wasn’t sure. Would that show a normal leukocyte differential? Is it CVID? Didn’t think CVID would have absent germinal centers in lymph nodes. What else could this be?

lispectedwumbologist  CVID presents in adulthood so it's not CVID. CVID also doesn't have absent germinal centers in lymph nodes. My dude has Bruton's agammaglobinemia +1  
sympathetikey  What @lispected said. +  




I think I found what the disease was, though I honestly have no idea why they would test this rather than XLA. There’s a condition called Transient Hypogammaglobulinemia of Infancy. It presents w/low immunoglobulin levels post 6 months and can present w/small lymph nodes and tonsils in infancy BUT w/o any other findings of primary immunodeficiency including decreased counts.

Here’s an article about it: https://primaryimmune.org/about-primary-immunodeficiencies/specific-disease-types/transient-hypogammaglobulinemia-of-infancy/

You definitely don’t need to know the disease to get the correct answer since the link of lack of immunoglobulins would clue you into the lack of germinal centers, but I think this is more likely than XLA since every source I read implies that B cell counts are near 0 in the classic presentation (unless I’m missing a reason why leukocyte count w/diff wouldn’t show a significant decrease in lymphocytes due to near-zero B cells). Just wanted to put this here in case other people later came wondering, though I may still be wrong.

keycompany  I think we are all overthinking this question (and it is remarkably simple). I don't think this question is testing us on any of the UFAP immunodeficiency syndromes, per say. What they are really asking is "what would also be absent if there are no immunoglobulins and everything else is normal"? The answer is absent germinal centers b/c that's where Ig is synthesized. I think they put all other aspects of the history in this stem just to help you rule out any of the other answers. +1