Nephrology and electrolytes: osmolal gap in toxic alcohols: practice question

A General 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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General Medicine · Nephrology and electrolytes: osmolal gap in toxic alcohols

Sample item

A man is brought in confused with a high anion gap metabolic acidosis. His measured serum osmolality exceeds the calculated osmolality by 35 mOsm/kg. How is this difference best interpreted?

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

The answer and the reasoning

C. An unmeasured small osmole such as a toxic alcohol

A raised osmolal gap means a low molecular weight osmole is present that the formula does not include, such as methanol, ethylene glycol, isopropanol or ethanol. In a high anion gap acidosis it suggests toxic alcohol ingestion early, before the alcohols are metabolised to acids and the gap falls.

Option by option

  • A. Severe uraemia with retained urea

    Urea is included in the osmolality formula and so does not create a gap.

  • B. Hyperlipidaemia causing an artefactual sodium reading

    Hyperlipidaemia affects measured sodium, not the difference between measured and calculated osmolality.

  • C. An unmeasured small osmole such as a toxic alcohol (correct)

    Correct. A large gap means an unmeasured osmole, typically a toxic alcohol.

  • D. Lactic acid accumulation from tissue hypoxia

    Lactate raises the anion gap but is already counted and 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.

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Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is C, An unmeasured small osmole such as a toxic alcohol. A raised osmolal gap means a low molecular weight osmole is present that the formula does not include, such as methanol, ethylene glycol, isopropanol or ethanol.

Why is "Severe uraemia with retained urea" not the answer?

Urea is included in the osmolality formula and so does not create a gap.

Why is "Hyperlipidaemia causing an artefactual sodium reading" not the answer?

Hyperlipidaemia affects measured sodium, not the difference between measured and calculated osmolality.

Why is "Lactic acid accumulation from tissue hypoxia" not the answer?

Lactate raises the anion gap but is already counted and does not raise the osmolal gap.

What concept does this question test?

It tests Nephrology and electrolytes: osmolal gap in toxic alcohols in General Medicine. In a high anion gap acidosis it suggests toxic alcohol ingestion early, before the alcohols are metabolised to acids and the gap falls.

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 Nephrology and electrolytes: osmolal gap in toxic alcohols?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General 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.