Purine catabolism: xanthinuria: 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 catabolism: xanthinuria

Sample item

A 30-year-old man has recurrent renal stones. Serum uric acid is very low at 0.5 mg/dL, and the urine contains high levels of xanthine and hypoxanthine. Which enzyme is deficient?

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

The answer and the reasoning

C. Xanthine oxidase

Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid, so deficiency lowers serum and urinary urate and increases xanthine excretion. Xanthine is poorly soluble and may form radiolucent stones. Treatment is high fluid intake, and allopurinol is not helpful.

Option by option

  • A. Adenosine deaminase

    Adenosine deaminase deficiency causes SCID and does not produce xanthine stones.

  • B. Glucose 6-phosphatase

    Glucose 6-phosphatase deficiency raises uric acid, since lactate competes with urate excretion.

  • C. Xanthine oxidase (correct)

    Correct. The last two steps of urate formation are blocked.

  • D. Phosphoribosyl pyrophosphate synthetase

    PRPP synthetase superactivity raises uric acid synthesis and causes gout.

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 C, Xanthine oxidase. Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid, so deficiency lowers serum and urinary urate and increases xanthine excretion.

Why is "Adenosine deaminase" not the answer?

Adenosine deaminase deficiency causes SCID and does not produce xanthine stones.

Why is "Glucose 6-phosphatase" not the answer?

Glucose 6-phosphatase deficiency raises uric acid, since lactate competes with urate excretion.

Why is "Phosphoribosyl pyrophosphate synthetase" not the answer?

PRPP synthetase superactivity raises uric acid synthesis and causes gout.

What concept does this question test?

It tests Purine catabolism: xanthinuria in Biochemistry. Xanthine is poorly soluble and may form radiolucent stones. Treatment is high fluid intake, and allopurinol is not helpful.

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 catabolism: xanthinuria?

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.