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 newborn has a persistently raised blood level of an aromatic amino acid on screening. The urinary pterin profile is normal, and the level falls substantially after a trial of sapropterin (tetrahydrobiopterin). Which enzyme is deficient, retaining some residual activity?

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

The answer and the reasoning

A. Phenylalanine hydroxylase

A normal pterin profile excludes a defect of cofactor synthesis or recycling. Tetrahydrobiopterin responsiveness indicates a hydroxylase with some residual function, as the cofactor can stabilise the mutant enzyme and increase its activity. Dihydropteridine reductase deficiency would show an abnormal pterin profile and neurotransmitter deficiency.

Option by option

  • A. Phenylalanine hydroxylase (correct)

    Correct. A normal pterin profile with cofactor response points to a mutant enzyme with residual activity.

  • B. Tyrosine aminotransferase

    Tyrosine aminotransferase deficiency raises tyrosine and does not raise the screened amino acid.

  • C. Dihydropteridine reductase

    Dihydropteridine reductase deficiency gives an abnormal pterin profile and neurotransmitter deficiency.

  • D. Homogentisate oxidase

    Homogentisate oxidase deficiency does not raise the screened amino acid.

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, Phenylalanine hydroxylase. A normal pterin profile excludes a defect of cofactor synthesis or recycling.

Why is "Tyrosine aminotransferase" not the answer?

Tyrosine aminotransferase deficiency raises tyrosine and does not raise the screened amino acid.

Why is "Dihydropteridine reductase" not the answer?

Dihydropteridine reductase deficiency gives an abnormal pterin profile and neurotransmitter deficiency.

Why is "Homogentisate oxidase" not the answer?

Homogentisate oxidase deficiency does not raise the screened amino acid.

What concept does this question test?

It tests Amino acid metabolism: phenylalanine hydroxylase in Biochemistry. Tetrahydrobiopterin responsiveness indicates a hydroxylase with some residual function, as the cofactor can stabilise the mutant enzyme and increase its activity. Dihydropteridine reductase deficiency would show an abnormal pterin profile and neurotransmitter deficiency.

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.