Amino acid metabolism: pyridoxal phosphate: 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: pyridoxal phosphate

Sample item

A 40-year-old malnourished man on a regimen containing isoniazid develops numbness and tingling of both feet. Which biochemical change underlies this neuropathy?

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

The answer and the reasoning

C. Depletion of pyridoxal phosphate, the cofactor for transaminases and decarboxylases

Isoniazid reacts with pyridoxal phosphate to form an inactive hydrazone and increases pyridoxine excretion, so the active coenzyme falls. Pyridoxal phosphate is required for transamination, decarboxylation (for GABA, serotonin, dopamine and histamine) and heme synthesis, and its deficiency produces peripheral neuropathy. Malnourished patients and alcoholics are at special risk.

Option by option

  • A. Depletion of NAD+ because tryptophan is not absorbed

    NAD+ deficiency from poor tryptophan absorption describes Hartnup disease or pellagra, not isoniazid toxicity.

  • B. Depletion of adenosylcobalamin, the cofactor for methylmalonyl-CoA mutase

    Cobalamin deficiency is not produced by isoniazid.

  • C. Depletion of pyridoxal phosphate, the cofactor for transaminases and decarboxylases (correct)

    Correct. Isoniazid inactivates pyridoxal phosphate and impairs amino acid transformations in nerves.

  • D. Depletion of thiamine pyrophosphate, the cofactor for pyruvate dehydrogenase

    Thiamine deficiency causes beriberi, a different clinical syndrome, and isoniazid does not inactivate it.

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 C, Depletion of pyridoxal phosphate, the cofactor for transaminases and decarboxylases. Isoniazid reacts with pyridoxal phosphate to form an inactive hydrazone and increases pyridoxine excretion, so the active coenzyme falls.

Why is "Depletion of NAD+ because tryptophan is not absorbed" not the answer?

NAD+ deficiency from poor tryptophan absorption describes Hartnup disease or pellagra, not isoniazid toxicity.

Why is "Depletion of adenosylcobalamin, the cofactor for methylmalonyl-CoA mu…" not the answer?

Cobalamin deficiency is not produced by isoniazid.

Why is "Depletion of thiamine pyrophosphate, the cofactor for pyruvate dehydr…" not the answer?

Thiamine deficiency causes beriberi, a different clinical syndrome, and isoniazid does not inactivate it.

What concept does this question test?

It tests Amino acid metabolism: pyridoxal phosphate in Biochemistry. Pyridoxal phosphate is required for transamination, decarboxylation (for GABA, serotonin, dopamine and histamine) and heme synthesis, and its deficiency produces peripheral neuropathy. Malnourished patients and alcoholics are at special risk.

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: pyridoxal phosphate?

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.