Nucleotide metabolism: pentostatin and adenosine deaminase: practice question

A Biochemistry 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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Biochemistry · Nucleotide metabolism: pentostatin and adenosine deaminase

Sample item

A drug used in hairy cell leukaemia causes lymphocytes to accumulate deoxyadenosine and dATP by inhibiting an enzyme whose inherited deficiency causes severe combined immunodeficiency. Which enzyme does it inhibit?

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

The answer and the reasoning

D. Adenosine deaminase

Pentostatin inhibits adenosine deaminase, so deoxyadenosine is converted to dATP, which inhibits ribonucleotide reductase and is toxic to lymphocytes. The same toxic accumulation occurs in inherited ADA deficiency, which causes SCID. Purine nucleoside phosphorylase deficiency causes a T-cell defect, and HGPRT and xanthine oxidase act elsewhere in purine metabolism.

Option by option

  • A. Purine nucleoside phosphorylase

    PNP deficiency gives a milder T-cell immunodeficiency with accumulation of deoxyguanosine, and it is not the target of pentostatin.

  • B. Hypoxanthine-guanine phosphoribosyltransferase

    HGPRT is the salvage enzyme lacking in Lesch-Nyhan syndrome.

  • C. Xanthine oxidase

    Xanthine oxidase is the target of allopurinol and febuxostat in gout.

  • D. Adenosine deaminase (correct)

    Correct. Blocking adenosine deaminase raises deoxyadenosine and dATP, killing 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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Adenosine deaminase. Pentostatin inhibits adenosine deaminase, so deoxyadenosine is converted to dATP, which inhibits ribonucleotide reductase and is toxic to lymphocytes.

Why is "Purine nucleoside phosphorylase" not the answer?

PNP deficiency gives a milder T-cell immunodeficiency with accumulation of deoxyguanosine, and it is not the target of pentostatin.

Why is "Hypoxanthine-guanine phosphoribosyltransferase" not the answer?

HGPRT is the salvage enzyme lacking in Lesch-Nyhan syndrome.

Why is "Xanthine oxidase" not the answer?

Xanthine oxidase is the target of allopurinol and febuxostat in gout.

What concept does this question test?

It tests Nucleotide metabolism: pentostatin and adenosine deaminase in Biochemistry. The same toxic accumulation occurs in inherited ADA deficiency, which causes SCID. Purine nucleoside phosphorylase deficiency causes a T-cell defect, and HGPRT and xanthine oxidase act elsewhere in purine metabolism.

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 Nucleotide metabolism: pentostatin and adenosine deaminase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.