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

In an inborn error, a non-essential aromatic amino acid can no longer be made from the essential one that accumulates, so the non-essential one becomes conditionally essential. Which enzyme is deficient?

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

The answer and the reasoning

D. Phenylalanine hydroxylase

Hydroxylation of phenylalanine is the only route to tyrosine, so when it fails tyrosine becomes essential in the diet. Tyrosine is also the precursor of catecholamines, thyroid hormones and melanin. Defects further down the tyrosine pathway do not remove tyrosine synthesis.

Option by option

  • A. Homogentisate oxidase

    Homogentisate oxidase acts in tyrosine breakdown and does not stop its synthesis.

  • B. Tyrosine aminotransferase

    Tyrosine aminotransferase deficiency raises tyrosine and causes tyrosinaemia type II.

  • C. Tyrosinase

    Tyrosinase acts within melanocytes to make melanin and its loss causes albinism.

  • D. Phenylalanine hydroxylase (correct)

    Correct. Blocking the conversion removes the body's only source of tyrosine apart from diet.

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 D, Phenylalanine hydroxylase. Hydroxylation of phenylalanine is the only route to tyrosine, so when it fails tyrosine becomes essential in the diet.

Why is "Homogentisate oxidase" not the answer?

Homogentisate oxidase acts in tyrosine breakdown and does not stop its synthesis.

Why is "Tyrosine aminotransferase" not the answer?

Tyrosine aminotransferase deficiency raises tyrosine and causes tyrosinaemia type II.

Why is "Tyrosinase" not the answer?

Tyrosinase acts within melanocytes to make melanin and its loss causes albinism.

What concept does this question test?

It tests Amino acid metabolism: phenylalanine hydroxylase in Biochemistry. Tyrosine is also the precursor of catecholamines, thyroid hormones and melanin. Defects further down the tyrosine pathway do not remove tyrosine synthesis.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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.