Vitamins: pyridoxine and isoniazid: 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 · Vitamins: pyridoxine and isoniazid

Sample item

A 50-year-old man on treatment for tuberculosis develops numbness and tingling of both feet. Deficiency of which vitamin is responsible, and by what mechanism?

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

The answer and the reasoning

C. Pyridoxine, because isoniazid binds pyridoxal phosphate and inhibits pyridoxal kinase

Isoniazid forms hydrazones with pyridoxal and inhibits pyridoxal kinase, depleting pyridoxal phosphate, the cofactor for transaminases and decarboxylases. This causes peripheral neuropathy, which is why pyridoxine is given to patients at high risk, such as those who are malnourished, diabetic, alcoholic or pregnant.

Option by option

  • A. Riboflavin, because isoniazid inhibits flavokinase

    Isoniazid is not known to inhibit flavokinase, and riboflavin deficiency causes stomatitis.

  • B. Folate, because isoniazid inhibits dihydrofolate reductase

    Dihydrofolate reductase is inhibited by methotrexate and trimethoprim, not isoniazid.

  • C. Pyridoxine, because isoniazid binds pyridoxal phosphate and inhibits pyridoxal kinase (correct)

    Isoniazid depletes pyridoxal phosphate, causing the neuropathy that pyridoxine prevents.

  • D. Thiamine, because isoniazid inhibits transketolase directly

    Isoniazid does not directly inhibit transketolase, and thiamine deficiency has a different pattern.

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, Pyridoxine, because isoniazid binds pyridoxal phosphate and inhibits pyridoxal kinase. Isoniazid forms hydrazones with pyridoxal and inhibits pyridoxal kinase, depleting pyridoxal phosphate, the cofactor for transaminases and decarboxylases.

Why is "Riboflavin, because isoniazid inhibits flavokinase" not the answer?

Isoniazid is not known to inhibit flavokinase, and riboflavin deficiency causes stomatitis.

Why is "Folate, because isoniazid inhibits dihydrofolate reductase" not the answer?

Dihydrofolate reductase is inhibited by methotrexate and trimethoprim, not isoniazid.

Why is "Thiamine, because isoniazid inhibits transketolase directly" not the answer?

Isoniazid does not directly inhibit transketolase, and thiamine deficiency has a different pattern.

What concept does this question test?

It tests Vitamins: pyridoxine and isoniazid in Biochemistry. This causes peripheral neuropathy, which is why pyridoxine is given to patients at high risk, such as those who are malnourished, diabetic, alcoholic or pregnant.

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 Vitamins: pyridoxine and isoniazid?

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.