Clinical biochemistry: 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 · Clinical biochemistry: osmolal gap

Sample item

A 30-year-old man is drowsy with visual blurring and metabolic acidosis. Measured serum osmolality is 330 mOsm/kg. Sodium is 140 mmol/L, glucose 90 mg/dL and blood urea nitrogen 14 mg/dL. What is the osmolal gap and its likely cause?

A0 mOsm/kg, lactic acidosisB10 mOsm/kg, diabetic ketoacidosisC40 mOsm/kg, uraemic acidosisD40 mOsm/kg, toxic alcohol ingestion such as methanol

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

The answer and the reasoning

D. 40 mOsm/kg, toxic alcohol ingestion such as methanol

Calculated osmolality = 2 x Na + glucose/18 + BUN/2.8 = 280 + 5 + 5 = 290 mOsm/kg. The osmolal gap is 330 - 290 = 40 mOsm/kg, well above the usual limit of about 10. Unmeasured small osmoles such as methanol or ethylene glycol cause a raised gap with high anion gap acidosis, and fomepizole is the treatment of choice.

Option by option

A. 0 mOsm/kg, lactic acidosis
A gap of zero is incorrect for these figures, and lactic acidosis usually causes little or no osmolal gap.
B. 10 mOsm/kg, diabetic ketoacidosis
The gap is 40 rather than 10, and DKA does not typically give such a large gap.
C. 40 mOsm/kg, uraemic acidosis
Uraemia is already included in the calculation through urea, so it does not generate this gap.
D. 40 mOsm/kg, toxic alcohol ingestion such as methanol (correct)
Correct. A gap of 40 with visual symptoms points to 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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, 40 mOsm/kg, toxic alcohol ingestion such as methanol. Calculated osmolality = 2 x Na + glucose/18 + BUN/2.

Why is "0 mOsm/kg, lactic acidosis" not the answer?

A gap of zero is incorrect for these figures, and lactic acidosis usually causes little or no osmolal gap.

Why is "10 mOsm/kg, diabetic ketoacidosis" not the answer?

The gap is 40 rather than 10, and DKA does not typically give such a large gap.

Why is "40 mOsm/kg, uraemic acidosis" not the answer?

Uraemia is already included in the calculation through urea, so it does not generate this gap.

What concept does this question test?

It tests Clinical biochemistry: osmolal gap in Biochemistry. 8 = 280 + 5 + 5 = 290 mOsm/kg. The osmolal gap is 330 - 290 = 40 mOsm/kg, well above the usual limit of about 10. Unmeasured small osmoles such as methanol or ethylene glycol cause a raised gap with high anion gap acidosis, and fomepizole is the treatment of choice.

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 Clinical biochemistry: 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.