Immunopathology: adenosine deaminase deficiency: 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 · Immunopathology: adenosine deaminase deficiency

Sample item

An 8-month-old infant has failure to thrive, oral candidiasis, Pneumocystis pneumonia and very low lymphocyte counts, with absent thymic shadow. The condition is autosomal recessive. Which mechanism underlies the lymphopenia?

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

The answer and the reasoning

D. Accumulation of deoxyadenosine metabolites toxic to lymphocytes

Adenosine deaminase deficiency is a cause of autosomal recessive SCID. Build-up of deoxyadenosine and dATP inhibits ribonucleotide reductase and DNA synthesis, killing developing T and B cells. Other SCID causes include common gamma chain defects (X-linked) and RAG mutations.

Option by option

  • A. Failure of B cells to switch from IgM to IgG

    Failure of class switching is the defect in hyper-IgM syndrome.

  • B. Defective CD40 ligand on helper T cells

    CD40 ligand deficiency gives hyper-IgM syndrome rather than profound lymphopenia.

  • C. Deficient NADPH oxidase in phagocytes

    NADPH oxidase deficiency causes chronic granulomatous disease with normal lymphocytes.

  • D. Accumulation of deoxyadenosine metabolites toxic to lymphocytes (correct)

    Correct. Toxic purine metabolites selectively destroy lymphocytes.

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 D, Accumulation of deoxyadenosine metabolites toxic to lymphocytes. Adenosine deaminase deficiency is a cause of autosomal recessive SCID.

Why is "Failure of B cells to switch from IgM to IgG" not the answer?

Failure of class switching is the defect in hyper-IgM syndrome.

Why is "Defective CD40 ligand on helper T cells" not the answer?

CD40 ligand deficiency gives hyper-IgM syndrome rather than profound lymphopenia.

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

NADPH oxidase deficiency causes chronic granulomatous disease with normal lymphocytes.

What concept does this question test?

It tests Immunopathology: adenosine deaminase deficiency in Pathology. Build-up of deoxyadenosine and dATP inhibits ribonucleotide reductase and DNA synthesis, killing developing T and B cells. Other SCID causes include common gamma chain defects (X-linked) and RAG mutations.

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 Immunopathology: adenosine deaminase deficiency?

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.