Immunodeficiency: X-linked agammaglobulinaemia: practice question

A Pathology 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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Pathology · Immunodeficiency: X-linked agammaglobulinaemia

Sample item

An 8-month-old boy has recurrent otitis media and pneumonia with encapsulated bacteria. Examination shows absent tonsils and no palpable nodes. Serum IgG, IgA and IgM are all very low and CD19+ B cells are under 1%, with normal T-cell counts. Which defect is responsible?

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

The answer and the reasoning

A. Mutation of Bruton tyrosine kinase, blocking pre-B to mature B cell maturation

X-linked agammaglobulinaemia arises from BTK mutations, so B cells arrest at the pre-B stage and all immunoglobulin classes are very low. Symptoms start after maternal IgG wanes at about 6 months, with sinopulmonary infections by encapsulated bacteria. Treatment is lifelong immunoglobulin replacement.

Option by option

  • A. Mutation of Bruton tyrosine kinase, blocking pre-B to mature B cell maturation (correct)

    Correct. Absent B cells with low immunoglobulins of all classes and normal T cells indicates BTK deficiency.

  • B. Mutation of the WAS gene

    WAS mutation causes Wiskott-Aldrich syndrome with eczema and thrombocytopenia.

  • C. Deficiency of adenosine deaminase

    ADA deficiency causes severe combined immunodeficiency with T-cell loss and lymphopenia.

  • D. Mutation of CD40 ligand, preventing class switching

    CD40L deficiency causes hyper-IgM syndrome with normal or high IgM and normal B cell counts.

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 Pathology question

What is the answer to this Pathology question?

The best answer is A, Mutation of Bruton tyrosine kinase, blocking pre-B to mature B cell maturation. X-linked agammaglobulinaemia arises from BTK mutations, so B cells arrest at the pre-B stage and all immunoglobulin classes are very low.

Why is "Mutation of the WAS gene" not the answer?

WAS mutation causes Wiskott-Aldrich syndrome with eczema and thrombocytopenia.

Why is "Deficiency of adenosine deaminase" not the answer?

ADA deficiency causes severe combined immunodeficiency with T-cell loss and lymphopenia.

Why is "Mutation of CD40 ligand, preventing class switching" not the answer?

CD40L deficiency causes hyper-IgM syndrome with normal or high IgM and normal B cell counts.

What concept does this question test?

It tests Immunodeficiency: X-linked agammaglobulinaemia in Pathology. Symptoms start after maternal IgG wanes at about 6 months, with sinopulmonary infections by encapsulated bacteria. Treatment is lifelong immunoglobulin replacement.

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 Immunodeficiency: X-linked agammaglobulinaemia?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.