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NBME 18 Answers

nbme18/Block 1/Question#40 (reveal difficulty score)
A 3 yo boy is brought to the physician for a ...
Complement activation ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–

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submitted by โˆ—motherhen(68)

This is describing Hyper IgM syndrome; without CD40 ligand, T cells are unable to provide the secondary signal to activate B cell class switching. This prevents IgM from switching into IgD, G, E and A, and thus an accumulation of IgM - hence the name "hyper IgM" - and deficiency in the rest (causing infections like S.pyogenes early in life, PCP, cryptosporidium and CMV).

Note in this question the cause of Giardia infection from lack of IgA (lack of mucosa/GI protection) must be distinguished from Selective IgA Deficiency. Lack of CD40L ligand defines it as Hyper IgM syndrome.

Answer choices:

  • Complement activation= IgM and IgG

  • Cross placenta= IgG

  • Attach eosinophil Fc receptor= IgE

  • Attach mast cells= IgE

  • Easy secretion across mucosa= IgA

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jer040512  To further add: IgG opsonizes +3

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