Nucleotide metabolism: xanthine oxidase in reperfusion injury: 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: xanthine oxidase in reperfusion injury

Sample item

In ischaemic tissue ATP is degraded and hypoxanthine accumulates. When blood flow returns, why is free radical injury produced?

AUricase converts the accumulated urate to allantoin and hydrogen peroxideBXanthine oxidase oxidises hypoxanthine using oxygen and releases superoxideCAdenosine deaminase converts hypoxanthine to uric acid without oxygenDHypoxanthine is salvaged to IMP and uses up all PRPP

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

The answer and the reasoning

B. Xanthine oxidase oxidises hypoxanthine using oxygen and releases superoxide

During ischaemia xanthine dehydrogenase is converted to the oxidase form while ATP breaks down to hypoxanthine. On reperfusion, oxygen allows xanthine oxidase to convert hypoxanthine to xanthine and urate, generating superoxide and hydrogen peroxide.

Option by option

A. Uricase converts the accumulated urate to allantoin and hydrogen peroxide
Humans lack functional uricase, so this step does not occur.
B. Xanthine oxidase oxidises hypoxanthine using oxygen and releases superoxide (correct)
Correct. Reintroduced oxygen drives xanthine oxidase and the release of reactive oxygen species.
C. Adenosine deaminase converts hypoxanthine to uric acid without oxygen
Adenosine deaminase acts on adenosine, not hypoxanthine, and the oxidation to urate needs oxygen.
D. Hypoxanthine is salvaged to IMP and uses up all PRPP
Salvage to IMP needs PRPP but does not generate radicals and would help restore ATP.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Xanthine oxidase oxidises hypoxanthine using oxygen and releases superoxide. During ischaemia xanthine dehydrogenase is converted to the oxidase form while ATP breaks down to hypoxanthine.

Why is "Uricase converts the accumulated urate to allantoin and hydrogen pero…" not the answer?

Humans lack functional uricase, so this step does not occur.

Why is "Adenosine deaminase converts hypoxanthine to uric acid without oxygen" not the answer?

Adenosine deaminase acts on adenosine, not hypoxanthine, and the oxidation to urate needs oxygen.

Why is "Hypoxanthine is salvaged to IMP and uses up all PRPP" not the answer?

Salvage to IMP needs PRPP but does not generate radicals and would help restore ATP.

What concept does this question test?

It tests Nucleotide metabolism: xanthine oxidase in reperfusion injury in Biochemistry. On reperfusion, oxygen allows xanthine oxidase to convert hypoxanthine to xanthine and urate, generating superoxide and hydrogen peroxide.

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: xanthine oxidase in reperfusion injury?

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.