Renal and acid-base: osmolal gap: practice question

A Physiology 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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Physiology · Renal and acid-base: osmolal gap

Sample item

A man brought in with blurred vision and confusion after drinking an unknown liquid has a high anion gap metabolic acidosis. Measured serum osmolality is 330 mOsm/kg, whereas the value calculated from sodium, glucose and blood urea nitrogen is 290. What does the gap of 40 indicate?

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

The answer and the reasoning

B. An unmeasured osmotically active toxic alcohol such as methanol

Calculated osmolality is 2 x sodium plus glucose/18 plus blood urea nitrogen/2.8. A measured value more than about 10 above this indicates an unmeasured osmotically active solute, such as methanol, ethylene glycol or isopropanol. Methanol typically causes visual symptoms and anion gap acidosis from formic acid.

Option by option

  • A. Hyperglycaemia, which is not in the calculation

    Glucose is part of the calculation, so it cannot explain the gap.

  • B. An unmeasured osmotically active toxic alcohol such as methanol (correct)

    Correct. A large osmolal gap with anion gap acidosis suggests a toxic alcohol.

  • C. Marked hypernatraemia, which is not in the calculation

    Sodium is the main term in the calculation, so it cannot explain a gap.

  • D. Excess urea, which is not in the calculation

    Urea is already included in the calculated osmolality.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, An unmeasured osmotically active toxic alcohol such as methanol. Calculated osmolality is 2 x sodium plus glucose/18 plus blood urea nitrogen/2.

Why is "Hyperglycaemia, which is not in the calculation" not the answer?

Glucose is part of the calculation, so it cannot explain the gap.

Why is "Marked hypernatraemia, which is not in the calculation" not the answer?

Sodium is the main term in the calculation, so it cannot explain a gap.

Why is "Excess urea, which is not in the calculation" not the answer?

Urea is already included in the calculated osmolality.

What concept does this question test?

It tests Renal and acid-base: osmolal gap in Physiology. 8. A measured value more than about 10 above this indicates an unmeasured osmotically active solute, such as methanol, ethylene glycol or isopropanol. Methanol typically causes visual symptoms and anion gap acidosis from formic acid.

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 Renal and acid-base: osmolal gap?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.