Toxicology: practice question

A Forensic Medicine 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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Forensic Medicine · Toxicology

Sample item

After drinking illicit liquor a man has blurred vision with a snowstorm appearance and severe metabolic acidosis. Which substance is mainly responsible for the ocular toxicity?

AFormic acid, a metabolite of methanolBOxalic acidCAcetaldehydeDUnmetabolised methanol itself

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

The answer and the reasoning

A. Formic acid, a metabolite of methanol

Methanol is oxidised by alcohol dehydrogenase to formaldehyde and then to formic acid, which inhibits cytochrome oxidase, produces anion-gap acidosis and damages the optic nerve. Fomepizole or ethanol blocks alcohol dehydrogenase and prevents toxic metabolite formation.

Option by option

A. Formic acid, a metabolite of methanol (correct)
Correct. Formic acid causes the acidosis and optic nerve toxicity.
B. Oxalic acid
Oxalic acid is a toxic metabolite of ethylene glycol, causing renal damage.
C. Acetaldehyde
Acetaldehyde is the metabolite of ethanol and causes flushing and hangover effects.
D. Unmetabolised methanol itself
Methanol itself is only mildly intoxicating, as the major toxicity depends on its metabolites.

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.

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is A, Formic acid, a metabolite of methanol. Methanol is oxidised by alcohol dehydrogenase to formaldehyde and then to formic acid, which inhibits cytochrome oxidase, produces anion-gap acidosis and damages the optic nerve.

Why is "Oxalic acid" not the answer?

Oxalic acid is a toxic metabolite of ethylene glycol, causing renal damage.

Why is "Acetaldehyde" not the answer?

Acetaldehyde is the metabolite of ethanol and causes flushing and hangover effects.

Why is "Unmetabolised methanol itself" not the answer?

Methanol itself is only mildly intoxicating, as the major toxicity depends on its metabolites.

What concept does this question test?

It tests Toxicology in Forensic Medicine. Fomepizole or ethanol blocks alcohol dehydrogenase and prevents toxic metabolite formation.

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?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine 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.