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NBME 21 Answers

nbme21/Block 2/Question#32 (reveal difficulty score)
A 17-year-old boy has the acute onset of ...
Lymphocytes 🔍 / 📺 / 🌳 / 📖

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submitted by pparalpha(93)

EBV (HHV-4) infects B cell through CD21.

Atypical lymphocytes on peripheral blood smear (not infected B cells, but reactive cytotoxic T cells).

"Mononucleosis": + monospot test (antibodies detected by agglutination of sheep or horse RBCs)

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sbryant6  Atypical lymphocytes are CD8+ T cells, not CD4+. Remember that. +17
mangotango  I remember this because Infectious Mononucleosis is caused by a virus (mostly EBV, sometimes CMV) and MHC Class I functions to present endogenous antigens (e.g. viral or cytosolic proteins) to CD8+ T cells. In comparison, MHC Class II is more involved with presenting exogenous antigens (e.g. bacterial proteins) to CD4+ T cells. // FA 2019, pg 100 +3
amy  I get the atypical T cell idea, but can someone help me understand why monocyte is not elevated in mononucleosis???? BTW, there are both heterophil positive(EBV) AND heterophil negative(CMV) mononucleosis, so patient can have - monospot test and still have mononucleosis... +
drdoom  @amy mononucleosis ≠ monocytes. Mono-nucleosis refers to the increase of mono-nuclear cells (lymphocytes) in the blood, as opposed to polymorpho-nuclear or “segmented” white blood cells like neutrophils, basophils, or eosinophils—which all have their nuclei divided into 2 or more “lobes” (multilobar cells). “Mono-nucleosis” just refers to an increase in the blood of those cells with a “single, unlobulated/unsegmented” nucleus = lymphocytes. +1

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submitted by calvin_and_hobbes(2)

"In addition to slightly decreased platelet count..." — why are plts slightly low in EBV infection/mono?

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