Amino acid metabolism: HHH syndrome: 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 · Amino acid metabolism: HHH syndrome

Sample item

A 5-year-old boy has episodic vomiting and confusion after protein-rich meals, with progressive spastic paraparesis. Plasma ammonia is raised during episodes, and urine contains homocitrulline. Plasma shows raised levels of the amino acid that accepts carbamoyl phosphate in the cycle. Which defect is most likely?

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

The answer and the reasoning

A. Defective mitochondrial transporter of ornithine

In HHH syndrome (hyperammonaemia, hyperornithinaemia, homocitrullinuria) the carrier that brings ornithine into mitochondria is defective. Ornithine accumulates in the cytosol, intramitochondrial ornithine is short, carbamoyl phosphate is diverted onto lysine to form homocitrulline, and ammonia rises.

Option by option

  • A. Defective mitochondrial transporter of ornithine (correct)

    Correct. Ornithine cannot enter mitochondria, so it accumulates outside, and carbamoyl phosphate reacts with lysine to form homocitrulline.

  • B. Deficiency of ornithine aminotransferase

    Ornithine aminotransferase deficiency causes gyrate atrophy of the retina with raised ornithine but without the hyperammonaemic episodes.

  • C. Deficiency of arginase in the cytosol

    Arginase deficiency raises plasma arginine, and ornithine is not elevated in the same way.

  • D. Deficiency of ornithine transcarbamylase

    OTC deficiency lowers citrulline and raises orotic acid without homocitrullinuria.

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 A, Defective mitochondrial transporter of ornithine. In HHH syndrome (hyperammonaemia, hyperornithinaemia, homocitrullinuria) the carrier that brings ornithine into mitochondria is defective.

Why is "Deficiency of ornithine aminotransferase" not the answer?

Ornithine aminotransferase deficiency causes gyrate atrophy of the retina with raised ornithine but without the hyperammonaemic episodes.

Why is "Deficiency of arginase in the cytosol" not the answer?

Arginase deficiency raises plasma arginine, and ornithine is not elevated in the same way.

Why is "Deficiency of ornithine transcarbamylase" not the answer?

OTC deficiency lowers citrulline and raises orotic acid without homocitrullinuria.

What concept does this question test?

It tests Amino acid metabolism: HHH syndrome in Biochemistry. Ornithine accumulates in the cytosol, intramitochondrial ornithine is short, carbamoyl phosphate is diverted onto lysine to form homocitrulline, and ammonia rises.

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 Amino acid metabolism: HHH syndrome?

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.