Amino acid metabolism: 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

Sample item

Carcinoid tumour cells divert large amounts of tryptophan into serotonin synthesis. Which enzyme catalyses the rate-limiting step of this pathway?

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

The answer and the reasoning

A. Tryptophan hydroxylase

Tryptophan hydroxylase forms 5-hydroxytryptophan using tetrahydrobiopterin, and is rate-limiting. Decarboxylase then makes serotonin, which monoamine oxidase degrades to 5-HIAA. Tryptophan diversion can lead to niacin deficiency in carcinoid syndrome.

Option by option

  • A. Tryptophan hydroxylase (correct)

    Correct. Tryptophan hydroxylase controls the entry of tryptophan into serotonin synthesis.

  • B. Aromatic L-amino acid decarboxylase

    The decarboxylase acts second and is not rate-limiting.

  • C. Monoamine oxidase A

    Monoamine oxidase A degrades serotonin rather than making it.

  • D. Tryptophan 2,3-dioxygenase

    Tryptophan 2,3-dioxygenase starts the kynurenine pathway towards niacin.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Tryptophan hydroxylase. Tryptophan hydroxylase forms 5-hydroxytryptophan using tetrahydrobiopterin, and is rate-limiting.

Why is "Aromatic L-amino acid decarboxylase" not the answer?

The decarboxylase acts second and is not rate-limiting.

Why is "Monoamine oxidase A" not the answer?

Monoamine oxidase A degrades serotonin rather than making it.

Why is "Tryptophan 2,3-dioxygenase" not the answer?

Tryptophan 2,3-dioxygenase starts the kynurenine pathway towards niacin.

What concept does this question test?

It tests Amino acid metabolism in Biochemistry. Decarboxylase then makes serotonin, which monoamine oxidase degrades to 5-HIAA. Tryptophan diversion can lead to niacin deficiency in carcinoid syndrome.

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?

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.