Nucleotide metabolism: effect of allopurinol on purine excretion: 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.

Try it

Biochemistry · Nucleotide metabolism: effect of allopurinol on purine excretion

Sample item

A patient with gout is started on allopurinol. Serum uric acid falls, and a later urine analysis shows increased hypoxanthine and xanthine excretion. What explains the changed excretion pattern?

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

The answer and the reasoning

C. Blocked oxidation of hypoxanthine and xanthine to uric acid

Allopurinol and its metabolite oxypurinol inhibit xanthine oxidase, so hypoxanthine and xanthine accumulate and are excreted instead of uric acid. These oxypurines are more soluble than urate, so crystal deposition falls. Some hypoxanthine is salvaged, which also reduces de novo synthesis.

Option by option

  • A. Increased activity of the salvage pathway enzyme HGPRT

    Salvage activity is not the cause, and HGPRT is not increased by this drug.

  • B. Failure of PRPP synthesis

    PRPP synthesis is not inhibited by allopurinol, and less PRPP would not explain oxypurine excretion.

  • C. Blocked oxidation of hypoxanthine and xanthine to uric acid (correct)

    Correct. Inhibiting xanthine oxidase shifts excretion to the more soluble oxypurine precursors.

  • D. Increased degradation of uric acid to allantoin

    Humans lack uricase, so uric acid is not degraded to allantoin by allopurinol.

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 C, Blocked oxidation of hypoxanthine and xanthine to uric acid. Allopurinol and its metabolite oxypurinol inhibit xanthine oxidase, so hypoxanthine and xanthine accumulate and are excreted instead of uric acid.

Why is "Increased activity of the salvage pathway enzyme HGPRT" not the answer?

Salvage activity is not the cause, and HGPRT is not increased by this drug.

Why is "Failure of PRPP synthesis" not the answer?

PRPP synthesis is not inhibited by allopurinol, and less PRPP would not explain oxypurine excretion.

Why is "Increased degradation of uric acid to allantoin" not the answer?

Humans lack uricase, so uric acid is not degraded to allantoin by allopurinol.

What concept does this question test?

It tests Nucleotide metabolism: effect of allopurinol on purine excretion in Biochemistry. These oxypurines are more soluble than urate, so crystal deposition falls. Some hypoxanthine is salvaged, which also reduces de novo synthesis.

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 Nucleotide metabolism: effect of allopurinol on purine excretion?

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.