Amino acid metabolism: ornithine aminotransferase deficiency: 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: ornithine aminotransferase deficiency

Sample item

A 22-year-old has night blindness and progressively constricted visual fields. Fundoscopy shows sharply demarcated, scalloped areas of chorioretinal atrophy in the periphery. Plasma ornithine is 10 to 20 times normal, and ammonia is normal. Which enzyme is deficient?

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

The answer and the reasoning

C. Ornithine aminotransferase

Ornithine aminotransferase is a pyridoxal phosphate enzyme that converts ornithine to glutamate semialdehyde. Its deficiency causes gyrate atrophy of the choroid and retina with hyperornithinaemia but normal ammonia. Some patients respond to pyridoxine, and arginine restriction lowers ornithine.

Option by option

  • A. Ornithine transcarbamylase

    OTC deficiency gives hyperammonaemia with low citrulline and high orotic acid.

  • B. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency causes phenylketonuria with neurological problems and not retinal atrophy.

  • C. Ornithine aminotransferase (correct)

    Correct. Failure of this transaminase gives high ornithine and gyrate chorioretinal atrophy with normal ammonia.

  • D. Arginase

    Arginase deficiency causes raised arginine and spastic diplegia rather than retinal atrophy.

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 aminotransferase. Ornithine aminotransferase is a pyridoxal phosphate enzyme that converts ornithine to glutamate semialdehyde.

Why is "Ornithine transcarbamylase" not the answer?

OTC deficiency gives hyperammonaemia with low citrulline and high orotic acid.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria with neurological problems and not retinal atrophy.

Why is "Arginase" not the answer?

Arginase deficiency causes raised arginine and spastic diplegia rather than retinal atrophy.

What concept does this question test?

It tests Amino acid metabolism: ornithine aminotransferase deficiency in Biochemistry. Its deficiency causes gyrate atrophy of the choroid and retina with hyperornithinaemia but normal ammonia. Some patients respond to pyridoxine, and arginine restriction lowers ornithine.

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: ornithine aminotransferase deficiency?

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.