Urea cycle disorders: 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 · Urea cycle disorders

Sample item

A 3-day-old boy is lethargic with a very high plasma ammonia, low citrulline and high urinary orotic acid. His maternal uncle died in infancy. Which enzyme is deficient?

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

The answer and the reasoning

C. Ornithine transcarbamylase

Ornithine transcarbamylase deficiency is X-linked. Carbamoyl phosphate accumulates in mitochondria, leaks to the cytosol and enters pyrimidine synthesis, which raises urinary orotic acid. In carbamoyl phosphate synthetase I deficiency, orotic acid is low. Citrulline is low in both because it is made downstream.

Option by option

  • A. Arginase

    Arginase deficiency gives high arginine and a spastic diplegia in older children.

  • B. Carbamoyl phosphate synthetase I

    CPS-I deficiency has hyperammonaemia and low citrulline but low orotic acid, and is autosomal recessive.

  • C. Ornithine transcarbamylase (correct)

    Correct. X-linked inheritance, low citrulline and high orotic acid point to this enzyme.

  • D. Argininosuccinate synthetase

    Argininosuccinate synthetase deficiency (citrullinaemia) gives very high citrulline.

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 C, Ornithine transcarbamylase. Ornithine transcarbamylase deficiency is X-linked.

Why is "Arginase" not the answer?

Arginase deficiency gives high arginine and a spastic diplegia in older children.

Why is "Carbamoyl phosphate synthetase I" not the answer?

CPS-I deficiency has hyperammonaemia and low citrulline but low orotic acid, and is autosomal recessive.

Why is "Argininosuccinate synthetase" not the answer?

Argininosuccinate synthetase deficiency (citrullinaemia) gives very high citrulline.

What concept does this question test?

It tests Urea cycle disorders in Biochemistry. Carbamoyl phosphate accumulates in mitochondria, leaks to the cytosol and enters pyrimidine synthesis, which raises urinary orotic acid. In carbamoyl phosphate synthetase I deficiency, orotic acid is low. Citrulline is low in both because it is made downstream.

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 Urea cycle disorders?

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.