Purine salvage: APRT deficiency: 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: APRT deficiency

Sample item

A 6-year-old boy has recurrent radiolucent renal stones and haematuria. Serum uric acid is normal and there are no neurological problems. Urine microscopy shows round brown crystals with central spicules. Which enzyme is deficient?

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

The answer and the reasoning

B. Adenine phosphoribosyltransferase

Without adenine phosphoribosyltransferase, adenine cannot be salvaged to AMP and is oxidised by xanthine oxidase to 2,8-dihydroxyadenine, which forms radiolucent stones often mistaken for uric acid stones. Allopurinol reduces formation. Serum uric acid is normal because the pathway to urate is unaffected.

Option by option

  • A. Hypoxanthine-guanine phosphoribosyltransferase

    HGPRT deficiency causes Lesch-Nyhan syndrome with hyperuricaemia and neurological features.

  • B. Adenine phosphoribosyltransferase (correct)

    Correct. APRT deficiency produces 2,8-dihydroxyadenine stones.

  • C. Xanthine oxidase

    Xanthine oxidase deficiency gives low serum uric acid and xanthine stones.

  • D. Purine nucleoside phosphorylase

    PNP deficiency gives T-cell immunodeficiency with low uric acid.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Adenine phosphoribosyltransferase. Without adenine phosphoribosyltransferase, adenine cannot be salvaged to AMP and is oxidised by xanthine oxidase to 2,8-dihydroxyadenine, which forms radiolucent stones often mistaken for uric acid stones.

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

HGPRT deficiency causes Lesch-Nyhan syndrome with hyperuricaemia and neurological features.

Why is "Xanthine oxidase" not the answer?

Xanthine oxidase deficiency gives low serum uric acid and xanthine stones.

Why is "Purine nucleoside phosphorylase" not the answer?

PNP deficiency gives T-cell immunodeficiency with low uric acid.

What concept does this question test?

It tests Purine salvage: APRT deficiency in Biochemistry. Allopurinol reduces formation. Serum uric acid is normal because the pathway to urate is unaffected.

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 Purine salvage: APRT deficiency?

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.