Purine catabolism: tumour lysis 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 · Purine catabolism: tumour lysis syndrome

Sample item

A 10-year-old boy with Burkitt lymphoma has high uric acid and is about to receive chemotherapy. Rasburicase is planned. Which test must be done first, and why?

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

The answer and the reasoning

B. G6PD activity, because hydrogen peroxide from urate oxidase action can cause haemolysis

Humans lack urate oxidase, so uric acid is the end product of purine catabolism. Rasburicase is recombinant urate oxidase that converts uric acid to the far more soluble allantoin, producing hydrogen peroxide as a by-product. G6PD-deficient red cells lack NADPH to detoxify peroxide and may haemolyse or form methaemoglobin, so rasburicase is contraindicated.

Option by option

  • A. TPMT activity, because rasburicase is a thiopurine

    TPMT testing is for thiopurines such as azathioprine and 6-mercaptopurine.

  • B. G6PD activity, because hydrogen peroxide from urate oxidase action can cause haemolysis (correct)

    Correct. Hydrogen peroxide generation makes G6PD deficiency a contraindication.

  • C. DPD activity, because rasburicase is a fluoropyrimidine

    DPD testing is for fluoropyrimidines such as fluorouracil and capecitabine.

  • D. HGPRT activity, because rasburicase is a purine salvage drug

    Rasburicase is an enzyme that degrades urate, and does not depend on HGPRT.

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 B, G6PD activity, because hydrogen peroxide from urate oxidase action can cause haemolysis. Humans lack urate oxidase, so uric acid is the end product of purine catabolism.

Why is "TPMT activity, because rasburicase is a thiopurine" not the answer?

TPMT testing is for thiopurines such as azathioprine and 6-mercaptopurine.

Why is "DPD activity, because rasburicase is a fluoropyrimidine" not the answer?

DPD testing is for fluoropyrimidines such as fluorouracil and capecitabine.

Why is "HGPRT activity, because rasburicase is a purine salvage drug" not the answer?

Rasburicase is an enzyme that degrades urate, and does not depend on HGPRT.

What concept does this question test?

It tests Purine catabolism: tumour lysis syndrome in Biochemistry. Rasburicase is recombinant urate oxidase that converts uric acid to the far more soluble allantoin, producing hydrogen peroxide as a by-product. G6PD-deficient red cells lack NADPH to detoxify peroxide and may haemolyse or form methaemoglobin, so rasburicase is contraindicated.

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 catabolism: tumour lysis 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.