Amino acid metabolism: catecholamine synthesis: 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: catecholamine synthesis

Sample item

A 45-year-old man with a biochemically proven pheochromocytoma is given metyrosine before surgery to reduce catecholamine production. Which enzyme does it inhibit?

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

The answer and the reasoning

B. Tyrosine hydroxylase

Tyrosine hydroxylase converts tyrosine to DOPA using tetrahydrobiopterin and is the rate-limiting step of catecholamine synthesis. Metyrosine (alpha-methyltyrosine) competitively inhibits it and lowers catecholamine output in pheochromocytoma, as an adjunct to alpha-blockade. DOPA decarboxylase is inhibited by carbidopa and dopamine beta-hydroxylase by disulfiram.

Option by option

  • A. Phenylethanolamine N-methyltransferase

    Phenylethanolamine N-methyltransferase makes adrenaline from noradrenaline and is not the target of metyrosine.

  • B. Tyrosine hydroxylase (correct)

    Correct. Metyrosine inhibits the rate-limiting hydroxylation of tyrosine.

  • C. Dopamine beta-hydroxylase

    Dopamine beta-hydroxylase is a copper enzyme inhibited by disulfiram.

  • D. DOPA decarboxylase

    DOPA decarboxylase is inhibited by carbidopa, which is used with levodopa in Parkinson disease.

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, Tyrosine hydroxylase. Tyrosine hydroxylase converts tyrosine to DOPA using tetrahydrobiopterin and is the rate-limiting step of catecholamine synthesis.

Why is "Phenylethanolamine N-methyltransferase" not the answer?

Phenylethanolamine N-methyltransferase makes adrenaline from noradrenaline and is not the target of metyrosine.

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

Dopamine beta-hydroxylase is a copper enzyme inhibited by disulfiram.

Why is "DOPA decarboxylase" not the answer?

DOPA decarboxylase is inhibited by carbidopa, which is used with levodopa in Parkinson disease.

What concept does this question test?

It tests Amino acid metabolism: catecholamine synthesis in Biochemistry. Metyrosine (alpha-methyltyrosine) competitively inhibits it and lowers catecholamine output in pheochromocytoma, as an adjunct to alpha-blockade. DOPA decarboxylase is inhibited by carbidopa and dopamine beta-hydroxylase by disulfiram.

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: catecholamine synthesis?

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.