Toxicology: isoniazid: practice question

A Pharmacology 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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Pharmacology · Toxicology: isoniazid

Sample item

A young woman who intentionally took a large number of isoniazid tablets has recurrent seizures that do not respond to diazepam alone, and metabolic acidosis. Which agent is the specific antidote?

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

The answer and the reasoning

D. Intravenous pyridoxine

Isoniazid inhibits pyridoxal phosphokinase and forms hydrazones with pyridoxine, depleting pyridoxal phosphate needed for glutamic acid decarboxylase, so GABA synthesis falls and refractory seizures result. Intravenous pyridoxine, in a dose equal to the ingested amount of isoniazid in grams, is the specific antidote and acts synergistically with benzodiazepines. Phenytoin is ineffective.

Option by option

  • A. Intravenous phenytoin

    Phenytoin is ineffective in isoniazid-induced seizures.

  • B. Intravenous calcium gluconate

    Calcium gluconate treats hypocalcaemia and does not restore GABA synthesis.

  • C. Intravenous fomepizole

    Fomepizole treats toxic alcohol poisoning.

  • D. Intravenous pyridoxine (correct)

    Pyridoxine restores GABA synthesis and terminates isoniazid seizures.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Intravenous pyridoxine. Isoniazid inhibits pyridoxal phosphokinase and forms hydrazones with pyridoxine, depleting pyridoxal phosphate needed for glutamic acid decarboxylase, so GABA synthesis falls and refractory seizures result.

Why is "Intravenous phenytoin" not the answer?

Phenytoin is ineffective in isoniazid-induced seizures.

Why is "Intravenous calcium gluconate" not the answer?

Calcium gluconate treats hypocalcaemia and does not restore GABA synthesis.

Why is "Intravenous fomepizole" not the answer?

Fomepizole treats toxic alcohol poisoning.

What concept does this question test?

It tests Toxicology: isoniazid in Pharmacology. Intravenous pyridoxine, in a dose equal to the ingested amount of isoniazid in grams, is the specific antidote and acts synergistically with benzodiazepines. Phenytoin is ineffective.

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 Toxicology: isoniazid?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.