Blood and immunity: X-linked agammaglobulinaemia: practice question

A Physiology 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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Physiology · Blood and immunity: X-linked agammaglobulinaemia

Sample item

A 9-month-old boy has recurrent otitis media and pneumonia from encapsulated bacteria after maternal IgG has waned. Flow cytometry shows almost no CD19-positive cells and all immunoglobulin classes are very low. What is the basic defect?

AFailure of thymic development, with absent T cellsBDefective NADPH oxidase in phagocytesCDeficiency of complement component C3DFailure of B-cell maturation at the pre-B stage from a tyrosine kinase mutation

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

The answer and the reasoning

D. Failure of B-cell maturation at the pre-B stage from a tyrosine kinase mutation

In Bruton agammaglobulinaemia a mutation in Btk blocks B-cell development at the pre-B stage, so mature B cells and plasma cells are absent and immunoglobulins are very low. Infections begin after maternal IgG wanes at about 6 months. T-cell function is largely preserved.

Option by option

A. Failure of thymic development, with absent T cells
Absent T cells would cause opportunistic infections early, with B cells present.
B. Defective NADPH oxidase in phagocytes
Defective NADPH oxidase causes chronic granulomatous disease with catalase-positive infections.
C. Deficiency of complement component C3
C3 deficiency would not remove B cells and immunoglobulins.
D. Failure of B-cell maturation at the pre-B stage from a tyrosine kinase mutation (correct)
Correct. Btk mutation arrests B cells, so antibodies cannot be made.

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 Physiology question

What is the answer to this Physiology question?

The best answer is D, Failure of B-cell maturation at the pre-B stage from a tyrosine kinase mutation. In Bruton agammaglobulinaemia a mutation in Btk blocks B-cell development at the pre-B stage, so mature B cells and plasma cells are absent and immunoglobulins are very low.

Why is "Failure of thymic development, with absent T cells" not the answer?

Absent T cells would cause opportunistic infections early, with B cells present.

Why is "Defective NADPH oxidase in phagocytes" not the answer?

Defective NADPH oxidase causes chronic granulomatous disease with catalase-positive infections.

Why is "Deficiency of complement component C3" not the answer?

C3 deficiency would not remove B cells and immunoglobulins.

What concept does this question test?

It tests Blood and immunity: X-linked agammaglobulinaemia in Physiology. Infections begin after maternal IgG wanes at about 6 months. T-cell function is largely preserved.

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 Blood and immunity: X-linked agammaglobulinaemia?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.