Amino acid metabolism: phenylalanine hydroxylase: 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: phenylalanine hydroxylase

Sample item

A woman with an inherited disorder of aromatic amino acid metabolism stops her diet in pregnancy. Her infant, who does not carry the disorder, has microcephaly, congenital heart disease and growth restriction. Which maternal enzyme deficiency is responsible?

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

The answer and the reasoning

B. Phenylalanine hydroxylase

In maternal phenylketonuria, a high maternal level of phenylalanine crosses the placenta and is teratogenic to the fetus even when the fetus is unaffected. Strict dietary control before and throughout pregnancy prevents this. Consider it whenever a woman with phenylketonuria plans a pregnancy.

Option by option

  • A. Dihydrofolate reductase

    Dihydrofolate reductase deficiency does not give this teratogenic picture with an unaffected fetus.

  • B. Phenylalanine hydroxylase (correct)

    Correct. High maternal phenylalanine from the untreated deficiency is teratogenic.

  • C. Homogentisate oxidase

    Homogentisate oxidase deficiency, alkaptonuria, is not associated with fetal malformation.

  • D. Branched-chain ketoacid dehydrogenase

    Branched-chain ketoacid dehydrogenase deficiency presents with neonatal encephalopathy, not maternal teratogenicity.

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, Phenylalanine hydroxylase. In maternal phenylketonuria, a high maternal level of phenylalanine crosses the placenta and is teratogenic to the fetus even when the fetus is unaffected.

Why is "Dihydrofolate reductase" not the answer?

Dihydrofolate reductase deficiency does not give this teratogenic picture with an unaffected fetus.

Why is "Homogentisate oxidase" not the answer?

Homogentisate oxidase deficiency, alkaptonuria, is not associated with fetal malformation.

Why is "Branched-chain ketoacid dehydrogenase" not the answer?

Branched-chain ketoacid dehydrogenase deficiency presents with neonatal encephalopathy, not maternal teratogenicity.

What concept does this question test?

It tests Amino acid metabolism: phenylalanine hydroxylase in Biochemistry. Strict dietary control before and throughout pregnancy prevents this. Consider it whenever a woman with phenylketonuria plans a pregnancy.

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: phenylalanine hydroxylase?

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.