Nucleotide metabolism: convergence of purine catabolism at xanthine: 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: convergence of purine catabolism at xanthine

Sample item

Adenine nucleotides are degraded via inosine and hypoxanthine, whereas guanine nucleotides are degraded via guanosine and guanine. Which compound is formed from both routes just before uric acid?

AAllantoinBInosineCXanthineDHypoxanthine

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

The answer and the reasoning

C. Xanthine

Hypoxanthine is oxidised by xanthine oxidase to xanthine, and guanine is deaminated by guanase to xanthine. Xanthine is then oxidised by the same enzyme to uric acid, the final product in humans. Allantoin is made only in animals that have uricase.

Option by option

A. Allantoin
Allantoin is formed after uric acid in species with uricase, not before it.
B. Inosine
Inosine is an intermediate of the adenine branch only.
C. Xanthine (correct)
Correct. Both pathways pass through xanthine, which xanthine oxidase converts to uric acid.
D. Hypoxanthine
Hypoxanthine comes only from the adenine branch, whereas guanine gives xanthine directly.

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 C, Xanthine. Hypoxanthine is oxidised by xanthine oxidase to xanthine, and guanine is deaminated by guanase to xanthine.

Why is "Allantoin" not the answer?

Allantoin is formed after uric acid in species with uricase, not before it.

Why is "Inosine" not the answer?

Inosine is an intermediate of the adenine branch only.

Why is "Hypoxanthine" not the answer?

Hypoxanthine comes only from the adenine branch, whereas guanine gives xanthine directly.

What concept does this question test?

It tests Nucleotide metabolism: convergence of purine catabolism at xanthine in Biochemistry. Xanthine is then oxidised by the same enzyme to uric acid, the final product in humans. Allantoin is made only in animals that have uricase.

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: convergence of purine catabolism at xanthine?

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.