Nucleotide metabolism: neurological basis of Lesch-Nyhan syndrome: 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: neurological basis of Lesch-Nyhan syndrome

Sample item

A boy with HGPRT deficiency has a normal urate on allopurinol, yet his dystonia and self-biting continue. What is the best explanation?

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

The answer and the reasoning

D. The neurological features arise from basal ganglia dopaminergic dysfunction, not urate

Lowering urate prevents gout and stones but does not reverse the neurobehavioural syndrome, which is linked to reduced dopamine in the basal ganglia from the enzyme deficiency itself. Allopurinol therefore does not cure the neurological disease.

Option by option

  • A. Allopurinol blocks hypoxanthine salvage in the brain

    Allopurinol inhibits xanthine oxidase and does not block salvage enzymes; the brain lacks the salvage enzyme already.

  • B. Residual xanthine is neurotoxic and persists on allopurinol

    Xanthine accumulation causes stones in some patients but has not been shown to be the cause of the brain disease.

  • C. Allopurinol raises PRPP and worsens the neurological damage

    Allopurinol does not increase PRPP, and PRPP excess is not the proven cause of the brain disease.

  • D. The neurological features arise from basal ganglia dopaminergic dysfunction, not urate (correct)

    Correct. The brain disease stems from the enzyme deficiency and dopaminergic loss rather than from high urate.

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, The neurological features arise from basal ganglia dopaminergic dysfunction, not urate. Lowering urate prevents gout and stones but does not reverse the neurobehavioural syndrome, which is linked to reduced dopamine in the basal ganglia from the enzyme deficiency itself.

Why is "Allopurinol blocks hypoxanthine salvage in the brain" not the answer?

Allopurinol inhibits xanthine oxidase and does not block salvage enzymes; the brain lacks the salvage enzyme already.

Why is "Residual xanthine is neurotoxic and persists on allopurinol" not the answer?

Xanthine accumulation causes stones in some patients but has not been shown to be the cause of the brain disease.

Why is "Allopurinol raises PRPP and worsens the neurological damage" not the answer?

Allopurinol does not increase PRPP, and PRPP excess is not the proven cause of the brain disease.

What concept does this question test?

It tests Nucleotide metabolism: neurological basis of Lesch-Nyhan syndrome in Biochemistry. Allopurinol therefore does not cure the neurological disease.

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: neurological basis of Lesch-Nyhan syndrome?

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.