Immunology: hyper IgM syndrome: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Immunology: hyper IgM syndrome

Sample item

A 3-year-old boy has recurrent pyogenic infections and Pneumocystis pneumonia. Serum IgM is high, while IgG, IgA and IgE are very low, and B cell numbers are normal. Which defect is responsible?

ADefective CD40 ligand on activated T cells, preventing class switchingBMutation of the WAS gene affecting the actin cytoskeletonCMutation of the CYBB gene encoding an oxidase subunitDMutation of the BTK gene blocking pre-B cell maturation

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Defective CD40 ligand on activated T cells, preventing class switching

In X-linked hyper IgM syndrome, T cells lack functional CD40 ligand, so CD40 on B cells cannot signal for isotype switching and germinal centre formation. IgM remains high, other classes are low, and defective T cell help to macrophages predisposes to Pneumocystis and Cryptosporidium.

Option by option

A. Defective CD40 ligand on activated T cells, preventing class switching (correct)
Correct. CD40L-CD40 signalling is essential for class switch recombination.
B. Mutation of the WAS gene affecting the actin cytoskeleton
WAS mutation causes eczema and thrombocytopenia with small platelets.
C. Mutation of the CYBB gene encoding an oxidase subunit
CYBB defects cause chronic granulomatous disease with a normal immunoglobulin profile.
D. Mutation of the BTK gene blocking pre-B cell maturation
BTK defects cause agammaglobulinaemia with absent B cells and low IgM too.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Defective CD40 ligand on activated T cells, preventing class switching. In X-linked hyper IgM syndrome, T cells lack functional CD40 ligand, so CD40 on B cells cannot signal for isotype switching and germinal centre formation.

Why is "Mutation of the WAS gene affecting the actin cytoskeleton" not the answer?

WAS mutation causes eczema and thrombocytopenia with small platelets.

Why is "Mutation of the CYBB gene encoding an oxidase subunit" not the answer?

CYBB defects cause chronic granulomatous disease with a normal immunoglobulin profile.

Why is "Mutation of the BTK gene blocking pre-B cell maturation" not the answer?

BTK defects cause agammaglobulinaemia with absent B cells and low IgM too.

What concept does this question test?

It tests Immunology: hyper IgM syndrome in Microbiology. IgM remains high, other classes are low, and defective T cell help to macrophages predisposes to Pneumocystis and Cryptosporidium.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Immunology: hyper IgM syndrome?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.