Enzyme inhibition: methanol poisoning: 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 · Enzyme inhibition: methanol poisoning

Sample item

A 30-year-old man develops blurred vision and a high anion gap metabolic acidosis after drinking adulterated country liquor containing methanol. Fomepizole is given. How does it work?

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

The answer and the reasoning

D. Competitively inhibits alcohol dehydrogenase, preventing formation of formaldehyde and formic acid

Methanol is oxidised by alcohol dehydrogenase to formaldehyde and then formate, the toxin that injures the optic nerve and causes acidosis. Fomepizole binds alcohol dehydrogenase with much higher affinity than methanol, so the toxic metabolites are not formed, and ethanol acts the same way when fomepizole is unavailable. Haemodialysis removes methanol and formate in severe cases.

Option by option

  • A. Chelates formic acid in the circulation

    Fomepizole does not chelate formate; it acts at the first enzyme step.

  • B. Irreversibly inactivates aldehyde dehydrogenase

    Inhibiting aldehyde dehydrogenase is the action of disulfiram and would not stop toxic metabolite formation.

  • C. Activates the cytochrome P450 system to detoxify methanol

    Fomepizole does not induce P450 enzymes and does not detoxify methanol.

  • D. Competitively inhibits alcohol dehydrogenase, preventing formation of formaldehyde and formic acid (correct)

    Correct. A higher-affinity competitive inhibitor blocks activation of methanol.

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, Competitively inhibits alcohol dehydrogenase, preventing formation of formaldehyde and formic acid. Methanol is oxidised by alcohol dehydrogenase to formaldehyde and then formate, the toxin that injures the optic nerve and causes acidosis.

Why is "Chelates formic acid in the circulation" not the answer?

Fomepizole does not chelate formate; it acts at the first enzyme step.

Why is "Irreversibly inactivates aldehyde dehydrogenase" not the answer?

Inhibiting aldehyde dehydrogenase is the action of disulfiram and would not stop toxic metabolite formation.

Why is "Activates the cytochrome P450 system to detoxify methanol" not the answer?

Fomepizole does not induce P450 enzymes and does not detoxify methanol.

What concept does this question test?

It tests Enzyme inhibition: methanol poisoning in Biochemistry. Fomepizole binds alcohol dehydrogenase with much higher affinity than methanol, so the toxic metabolites are not formed, and ethanol acts the same way when fomepizole is unavailable. Haemodialysis removes methanol and formate in severe cases.

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 Enzyme inhibition: methanol poisoning?

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.