Toxicology: osmolal gap: 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 · Toxicology: osmolal gap

Sample item

A 30-year-old man is brought with confusion and flank pain after drinking an unknown liquid. Serum Na 140 mmol/L, glucose 90 mg/dL, urea nitrogen 14 mg/dL, measured osmolality 330 mOsm/kg, anion gap markedly raised, calcium low, and urine microscopy shows envelope-shaped crystals. What is the osmolal gap and the most likely agent?

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

The answer and the reasoning

A. About 40 mOsm/kg, ethylene glycol

Calculated osmolality = 2 x 140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290 mOsm/kg. The gap is 330 - 290 = 40 mOsm/kg, well above the normal value of under 10. Ethylene glycol is an osmotically active alcohol whose metabolites cause high anion gap acidosis, hypocalcaemia and calcium oxalate crystals; treatment is fomepizole and dialysis.

Option by option

  • A. About 40 mOsm/kg, ethylene glycol (correct)

    Correct. A large gap with oxalate crystals and hypocalcaemia indicates ethylene glycol.

  • B. About 10 mOsm/kg, methanol

    A gap of 10 is normal and the calculated gap here is 40.

  • C. About 40 mOsm/kg, metformin

    Metformin lactic acidosis produces an anion gap but not an osmolal gap or crystalluria.

  • D. About 40 mOsm/kg, salicylate

    Salicylates cause anion gap acidosis but no osmolal gap or oxalate crystals.

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 A, About 40 mOsm/kg, ethylene glycol. Calculated osmolality = 2 x 140 + 90/18 + 14/2.

Why is "About 10 mOsm/kg, methanol" not the answer?

A gap of 10 is normal and the calculated gap here is 40.

Why is "About 40 mOsm/kg, metformin" not the answer?

Metformin lactic acidosis produces an anion gap but not an osmolal gap or crystalluria.

Why is "About 40 mOsm/kg, salicylate" not the answer?

Salicylates cause anion gap acidosis but no osmolal gap or oxalate crystals.

What concept does this question test?

It tests Toxicology: osmolal gap in Biochemistry. 8 = 280 + 5 + 5 = 290 mOsm/kg. The gap is 330 - 290 = 40 mOsm/kg, well above the normal value of under 10. Ethylene glycol is an osmotically active alcohol whose metabolites cause high anion gap acidosis, hypocalcaemia and calcium oxalate crystals; treatment is fomepizole and dialysis.

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?

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.