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 child with fair skin and light hair has eczema and developmental delay, though both parents have brown hair. Melanocyte tyrosinase is structurally normal, but an accumulating amino acid competitively inhibits it. Which enzyme deficiency is the primary defect?

ATyrosinaseBTyrosine hydroxylaseCPhenylalanine hydroxylaseDDopamine beta-hydroxylase

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

The answer and the reasoning

C. Phenylalanine hydroxylase

The pigment dilution in phenylketonuria is secondary, because accumulated phenylalanine competitively inhibits melanocyte tyrosinase. The primary defect is therefore upstream, in the hepatic hydroxylase. Primary tyrosinase deficiency would be oculocutaneous albinism without developmental delay.

Option by option

A. Tyrosinase
Tyrosinase is structurally normal here; its deficiency would cause albinism without developmental delay.
B. Tyrosine hydroxylase
Tyrosine hydroxylase deficiency impairs dopamine synthesis and does not cause hypopigmentation.
C. Phenylalanine hydroxylase (correct)
Correct. The primary defect is the hepatic enzyme; inhibition of tyrosinase is secondary.
D. Dopamine beta-hydroxylase
Dopamine beta-hydroxylase deficiency causes orthostatic hypotension, not fair pigmentation.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Phenylalanine hydroxylase. The pigment dilution in phenylketonuria is secondary, because accumulated phenylalanine competitively inhibits melanocyte tyrosinase.

Why is "Tyrosinase" not the answer?

Tyrosinase is structurally normal here; its deficiency would cause albinism without developmental delay.

Why is "Tyrosine hydroxylase" not the answer?

Tyrosine hydroxylase deficiency impairs dopamine synthesis and does not cause hypopigmentation.

Why is "Dopamine beta-hydroxylase" not the answer?

Dopamine beta-hydroxylase deficiency causes orthostatic hypotension, not fair pigmentation.

What concept does this question test?

It tests Amino acid metabolism: phenylalanine hydroxylase in Biochemistry. The primary defect is therefore upstream, in the hepatic hydroxylase. Primary tyrosinase deficiency would be oculocutaneous albinism without developmental delay.

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.