Toxicology: osmolal gap in toxic alcohols: 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: osmolal gap in toxic alcohols

Sample item

A patient with suspected toxic alcohol ingestion has a high anion gap acidosis and a raised osmolal gap. When the patient presents late, why may the osmolal gap be normal?

AThe anion gap rises only when ethanol is presentBThe parent alcohol has already been metabolised to acids, which do not add to osmolalityCToxic alcohols never alter osmolalityDThe osmolal gap is raised only in salicylate poisoning

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

The answer and the reasoning

B. The parent alcohol has already been metabolised to acids, which do not add to osmolality

Methanol and ethylene glycol are osmotically active and raise the osmolal gap early. Later they are converted to formic acid or glycolic and oxalic acids, which raise the anion gap but contribute little osmolality. A normal gap therefore does not exclude late poisoning.

Option by option

A. The anion gap rises only when ethanol is present
The anion gap rises from their acid metabolites and not from ethanol.
B. The parent alcohol has already been metabolised to acids, which do not add to osmolality (correct)
Correct. Metabolism to acids lowers the osmolal gap and raises the anion gap.
C. Toxic alcohols never alter osmolality
Toxic alcohols are osmotically active and do raise the gap early.
D. The osmolal gap is raised only in salicylate poisoning
Salicylate poisoning does not raise the osmolal gap.

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 B, The parent alcohol has already been metabolised to acids, which do not add to osmolality. Methanol and ethylene glycol are osmotically active and raise the osmolal gap early.

Why is "The anion gap rises only when ethanol is present" not the answer?

The anion gap rises from their acid metabolites and not from ethanol.

Why is "Toxic alcohols never alter osmolality" not the answer?

Toxic alcohols are osmotically active and do raise the gap early.

Why is "The osmolal gap is raised only in salicylate poisoning" not the answer?

Salicylate poisoning does not raise the osmolal gap.

What concept does this question test?

It tests Toxicology: osmolal gap in toxic alcohols in Forensic Medicine. Later they are converted to formic acid or glycolic and oxalic acids, which raise the anion gap but contribute little osmolality. A normal gap therefore does not exclude late poisoning.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: osmolal gap in toxic alcohols?

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.