Pyrimidine synthesis: orotic aciduria: 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 · Pyrimidine synthesis: orotic aciduria

Sample item

An 8-month-old infant has failure to thrive and megaloblastic anaemia that does not respond to vitamin B12 or folate. Urine contains orotic acid crystals and ammonia is normal. What is the treatment of choice?

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

The answer and the reasoning

D. Oral uridine

Hereditary orotic aciduria is a defect of the bifunctional UMP synthase (orotate phosphoribosyltransferase and OMP decarboxylase). Uridine is converted to UMP by salvage and also inhibits carbamoyl phosphate synthetase II, reducing orotic acid production. Normal ammonia distinguishes it from ornithine transcarbamylase deficiency.

Option by option

  • A. Sodium benzoate with protein restriction

    Sodium benzoate lowers ammonia in urea cycle defects, but ammonia is normal here.

  • B. Cyanocobalamin

    B12 does not correct this defect, as the anaemia is refractory to B12.

  • C. Allopurinol

    Allopurinol would be considered for purine disorders and can even raise orotic acid.

  • D. Oral uridine (correct)

    Correct. Uridine bypasses the block and supplies pyrimidines.

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 D, Oral uridine. Hereditary orotic aciduria is a defect of the bifunctional UMP synthase (orotate phosphoribosyltransferase and OMP decarboxylase).

Why is "Sodium benzoate with protein restriction" not the answer?

Sodium benzoate lowers ammonia in urea cycle defects, but ammonia is normal here.

Why is "Cyanocobalamin" not the answer?

B12 does not correct this defect, as the anaemia is refractory to B12.

Why is "Allopurinol" not the answer?

Allopurinol would be considered for purine disorders and can even raise orotic acid.

What concept does this question test?

It tests Pyrimidine synthesis: orotic aciduria in Biochemistry. Uridine is converted to UMP by salvage and also inhibits carbamoyl phosphate synthetase II, reducing orotic acid production. Normal ammonia distinguishes it from ornithine transcarbamylase deficiency.

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 Pyrimidine synthesis: orotic aciduria?

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.