Immunology: adenosine deaminase deficiency: 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: adenosine deaminase deficiency

Sample item

An infant has severe recurrent infections, very low lymphocyte counts and flaring at the costochondral junctions on chest radiograph. The diagnosis is adenosine deaminase deficient severe combined immunodeficiency. Which metabolites accumulate and damage lymphocytes?

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

The answer and the reasoning

A. Deoxyadenosine and deoxyadenosine triphosphate

Without adenosine deaminase, deoxyadenosine accumulates and is converted to dATP, which is toxic to lymphocytes by inhibiting ribonucleotide reductase and DNA synthesis. This leads to a profound loss of T, B and NK cells. Deoxyguanosine accumulates in purine nucleoside phosphorylase deficiency, which mainly affects T cells.

Option by option

  • A. Deoxyadenosine and deoxyadenosine triphosphate (correct)

    Correct. dATP accumulation poisons developing lymphocytes in ADA deficiency.

  • B. Orotic acid and uridine

    Orotic acid accumulates in hereditary orotic aciduria and in ornithine transcarbamylase deficiency, not in ADA deficiency.

  • C. Homocysteine and methionine

    Homocysteine accumulation is a feature of homocystinuria and is unrelated to purine salvage.

  • D. Deoxyguanosine and deoxyguanosine triphosphate

    Deoxyguanosine and dGTP build up in purine nucleoside phosphorylase deficiency, which primarily affects T cells.

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 A, Deoxyadenosine and deoxyadenosine triphosphate. Without adenosine deaminase, deoxyadenosine accumulates and is converted to dATP, which is toxic to lymphocytes by inhibiting ribonucleotide reductase and DNA synthesis.

Why is "Orotic acid and uridine" not the answer?

Orotic acid accumulates in hereditary orotic aciduria and in ornithine transcarbamylase deficiency, not in ADA deficiency.

Why is "Homocysteine and methionine" not the answer?

Homocysteine accumulation is a feature of homocystinuria and is unrelated to purine salvage.

Why is "Deoxyguanosine and deoxyguanosine triphosphate" not the answer?

Deoxyguanosine and dGTP build up in purine nucleoside phosphorylase deficiency, which primarily affects T cells.

What concept does this question test?

It tests Immunology: adenosine deaminase deficiency in Microbiology. This leads to a profound loss of T, B and NK cells. Deoxyguanosine accumulates in purine nucleoside phosphorylase deficiency, which mainly affects T cells.

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: adenosine deaminase deficiency?

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.