Immunology: X-linked agammaglobulinaemia: 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: X-linked agammaglobulinaemia

Sample item

A 9-month-old boy has recurrent pneumococcal pneumonia and otitis media. Tonsils are not visible, and flow cytometry shows absent CD19-positive cells with low levels of all immunoglobulin classes. Which gene is defective?

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

The answer and the reasoning

C. BTK gene

Bruton agammaglobulinaemia is caused by a defect in Bruton tyrosine kinase, which arrests pre-B cell maturation, so mature B cells and all immunoglobulins are absent. CD40L defect gives hyper IgM with normal B cells, ADA deficiency gives SCID, and WAS gives eczema and thrombocytopenia.

Option by option

  • A. CD40 ligand gene

    CD40L deficiency has normal B cell numbers and high IgM.

  • B. WAS gene

    WAS defect causes eczema, microthrombocytopenia and infections.

  • C. BTK gene (correct)

    Correct. BTK deficiency causes absent B cells and panhypogammaglobulinaemia.

  • D. Adenosine deaminase gene

    ADA deficiency affects T cells as well and presents as SCID.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, BTK gene. Bruton agammaglobulinaemia is caused by a defect in Bruton tyrosine kinase, which arrests pre-B cell maturation, so mature B cells and all immunoglobulins are absent.

Why is "CD40 ligand gene" not the answer?

CD40L deficiency has normal B cell numbers and high IgM.

Why is "WAS gene" not the answer?

WAS defect causes eczema, microthrombocytopenia and infections.

Why is "Adenosine deaminase gene" not the answer?

ADA deficiency affects T cells as well and presents as SCID.

What concept does this question test?

It tests Immunology: X-linked agammaglobulinaemia in Microbiology. CD40L defect gives hyper IgM with normal B cells, ADA deficiency gives SCID, and WAS gives eczema and thrombocytopenia.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: X-linked agammaglobulinaemia?

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.