Amino acid metabolism: tryptophan load and vitamin B6: 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: tryptophan load and vitamin B6

Sample item

After an oral tryptophan load, a patient on long-term isoniazid excretes large amounts of xanthurenic acid in urine. Deficiency of which vitamin is suggested by this test?

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

The answer and the reasoning

D. Pyridoxine

Kynureninase, which converts kynurenine onward in tryptophan catabolism, needs pyridoxal phosphate. In deficiency, kynurenine is diverted to xanthurenic acid, which is excreted in excess after a tryptophan load. Isoniazid depletes pyridoxal phosphate, so supplementation is routine.

Option by option

  • A. Thiamine

    Thiamine deficiency affects the pyruvate and alpha-ketoglutarate dehydrogenase complexes and not tryptophan catabolism.

  • B. Niacin

    Niacin deficiency causes pellagra but does not block kynureninase.

  • C. Riboflavin

    Riboflavin is needed for FAD-dependent enzymes and does not cause this metabolite pattern.

  • D. Pyridoxine (correct)

    Correct. Kynureninase needs pyridoxal phosphate, so a block diverts kynurenine to xanthurenic 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 D, Pyridoxine. Kynureninase, which converts kynurenine onward in tryptophan catabolism, needs pyridoxal phosphate.

Why is "Thiamine" not the answer?

Thiamine deficiency affects the pyruvate and alpha-ketoglutarate dehydrogenase complexes and not tryptophan catabolism.

Why is "Niacin" not the answer?

Niacin deficiency causes pellagra but does not block kynureninase.

Why is "Riboflavin" not the answer?

Riboflavin is needed for FAD-dependent enzymes and does not cause this metabolite pattern.

What concept does this question test?

It tests Amino acid metabolism: tryptophan load and vitamin B6 in Biochemistry. In deficiency, kynurenine is diverted to xanthurenic acid, which is excreted in excess after a tryptophan load. Isoniazid depletes pyridoxal phosphate, so supplementation is routine.

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: tryptophan load and vitamin B6?

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.