Nucleotide metabolism: end products of pyrimidine catabolism: 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: end products of pyrimidine catabolism

Sample item

Rapid cell turnover during chemotherapy releases large amounts of both purines and pyrimidines. Why does pyrimidine breakdown not cause crystal deposition like uric acid does?

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

The answer and the reasoning

B. Its end products, beta-alanine and beta-aminoisobutyrate, are highly water-soluble

The pyrimidine ring is opened and degraded to beta-alanine from cytosine and uracil, and beta-aminoisobutyrate from thymine, with ammonia and carbon dioxide. These products are highly soluble, so pyrimidine catabolism does not cause gout. Purine breakdown ends at uric acid, which is poorly soluble.

Option by option

  • A. Pyrimidines are excreted unchanged in the urine without catabolism

    Pyrimidines are catabolised and not simply excreted unchanged.

  • B. Its end products, beta-alanine and beta-aminoisobutyrate, are highly water-soluble (correct)

    Correct. The soluble end products beta-alanine and beta-aminoisobutyrate do not crystallise.

  • C. Pyrimidines are completely recycled by salvage and never degraded

    Salvage recycles only part of the bases, and the remainder is degraded.

  • D. Pyrimidines are broken down to allantoin by uricase

    Uricase acts on uric acid in other mammals and is not involved in pyrimidine catabolism.

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, Its end products, beta-alanine and beta-aminoisobutyrate, are highly water-soluble. The pyrimidine ring is opened and degraded to beta-alanine from cytosine and uracil, and beta-aminoisobutyrate from thymine, with ammonia and carbon dioxide.

Why is "Pyrimidines are excreted unchanged in the urine without catabolism" not the answer?

Pyrimidines are catabolised and not simply excreted unchanged.

Why is "Pyrimidines are completely recycled by salvage and never degraded" not the answer?

Salvage recycles only part of the bases, and the remainder is degraded.

Why is "Pyrimidines are broken down to allantoin by uricase" not the answer?

Uricase acts on uric acid in other mammals and is not involved in pyrimidine catabolism.

What concept does this question test?

It tests Nucleotide metabolism: end products of pyrimidine catabolism in Biochemistry. These products are highly soluble, so pyrimidine catabolism does not cause gout. Purine breakdown ends at uric acid, which is poorly soluble.

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: end products of pyrimidine catabolism?

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.