Gastroenterology: stool osmotic gap: 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 · Gastroenterology: stool osmotic gap

Sample item

A patient has chronic watery diarrhoea that stops with fasting. Stool Na 40, K 30 mmol/L and stool osmolality 290 mOsm/kg. What does this indicate?

ANormal stoolBSecretory diarrhoeaCInflammatory diarrhoea onlyDOsmotic diarrhoea, as the osmotic gap is about 150

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

The answer and the reasoning

D. Osmotic diarrhoea, as the osmotic gap is about 150

Stool osmotic gap is 290 minus 2 times (Na plus K), which is 290 minus 140, giving 150. A gap above 125 indicates an osmotic cause such as lactose intolerance, laxatives or malabsorption, and diarrhoea stops with fasting. Secretory diarrhoea has a gap below 50 and persists on fasting.

Option by option

A. Normal stool
The gap is clearly abnormal.
B. Secretory diarrhoea
Secretory diarrhoea has a low gap and continues during fasting.
C. Inflammatory diarrhoea only
Inflammatory diarrhoea is identified by blood, leukocytes or calprotectin, not by the gap.
D. Osmotic diarrhoea, as the osmotic gap is about 150 (correct)
Correct. A gap above 125 indicates osmotic diarrhoea.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, Osmotic diarrhoea, as the osmotic gap is about 150. Stool osmotic gap is 290 minus 2 times (Na plus K), which is 290 minus 140, giving 150.

Why is "Normal stool" not the answer?

The gap is clearly abnormal.

Why is "Secretory diarrhoea" not the answer?

Secretory diarrhoea has a low gap and continues during fasting.

Why is "Inflammatory diarrhoea only" not the answer?

Inflammatory diarrhoea is identified by blood, leukocytes or calprotectin, not by the gap.

What concept does this question test?

It tests Gastroenterology: stool osmotic gap in General Medicine. A gap above 125 indicates an osmotic cause such as lactose intolerance, laxatives or malabsorption, and diarrhoea stops with fasting. Secretory diarrhoea has a gap below 50 and persists on fasting.

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 Gastroenterology: stool osmotic gap?

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.