Purine salvage: hypoxanthine-guanine phosphoribosyltransferase: 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 · Purine salvage: hypoxanthine-guanine phosphoribosyltransferase

Sample item

A boy with severe gout and renal stones is treated with allopurinol and his urate falls to normal. Dystonia, intellectual disability and self-injury continue. Which enzyme deficiency explains why the neurological features persist?

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

The answer and the reasoning

D. Hypoxanthine-guanine phosphoribosyltransferase

Allopurinol blocks xanthine oxidase and lowers urate, which relieves gout and stones. The brain disease of Lesch-Nyhan syndrome is due to the salvage defect itself, probably through basal ganglia dopamine dysfunction, and does not respond. Urate lowering treats only the metabolic consequences.

Option by option

  • A. Phosphoribosyl pyrophosphate synthetase

    PRPP synthetase superactivity causes gout and deafness, without self-mutilation.

  • B. Xanthine oxidase

    Xanthine oxidase is the target of allopurinol, so it is inhibited by treatment rather than deficient.

  • C. Adenosine deaminase

    Adenosine deaminase deficiency causes immunodeficiency, not self-injury and dystonia.

  • D. Hypoxanthine-guanine phosphoribosyltransferase (correct)

    Correct. The salvage defect itself damages the basal ganglia, so urate lowering does not help.

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, Hypoxanthine-guanine phosphoribosyltransferase. Allopurinol blocks xanthine oxidase and lowers urate, which relieves gout and stones.

Why is "Phosphoribosyl pyrophosphate synthetase" not the answer?

PRPP synthetase superactivity causes gout and deafness, without self-mutilation.

Why is "Xanthine oxidase" not the answer?

Xanthine oxidase is the target of allopurinol, so it is inhibited by treatment rather than deficient.

Why is "Adenosine deaminase" not the answer?

Adenosine deaminase deficiency causes immunodeficiency, not self-injury and dystonia.

What concept does this question test?

It tests Purine salvage: hypoxanthine-guanine phosphoribosyltransferase in Biochemistry. The brain disease of Lesch-Nyhan syndrome is due to the salvage defect itself, probably through basal ganglia dopamine dysfunction, and does not respond. Urate lowering treats only the metabolic consequences.

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 Purine salvage: hypoxanthine-guanine phosphoribosyltransferase?

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.