GI physiology: osmotic diarrhoea: 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 · GI physiology: osmotic diarrhoea

Sample item

A 17-year-old boy has bloating and watery stools after drinking milk. A stool sample has sodium 30 mmol/L and potassium 20 mmol/L, and plasma osmolality is 290 mOsm/kg. What is the stool osmotic gap and its interpretation?

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

The answer and the reasoning

A. 190 mOsm/kg, indicating osmotic diarrhoea

Stool osmotic gap is 290 minus 2 times the sum of stool sodium and potassium, because anions accompany the cations. Here 2 times (30 plus 20) is 100, and 290 minus 100 gives 190 mOsm/kg. A gap above about 100 to 125 means unmeasured osmoles such as unabsorbed lactose, as in lactase deficiency, while a gap below about 50 suggests secretory diarrhoea.

Option by option

  • A. 190 mOsm/kg, indicating osmotic diarrhoea (correct)

    The calculation gives 190, which is above 125 and indicates unabsorbed osmoles.

  • B. 240 mOsm/kg, indicating osmotic diarrhoea

    The value 240 results from not doubling the cation sum, which ignores the accompanying anions.

  • C. 50 mOsm/kg, indicating secretory diarrhoea

    The cation sum of 50 is not the gap, and the real gap points to osmotic diarrhoea.

  • D. 90 mOsm/kg, indicating secretory diarrhoea

    A value of 90 would be a miscalculation, and the real gap is not in the secretory range.

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 A, 190 mOsm/kg, indicating osmotic diarrhoea. Stool osmotic gap is 290 minus 2 times the sum of stool sodium and potassium, because anions accompany the cations.

Why is "240 mOsm/kg, indicating osmotic diarrhoea" not the answer?

The value 240 results from not doubling the cation sum, which ignores the accompanying anions.

Why is "50 mOsm/kg, indicating secretory diarrhoea" not the answer?

The cation sum of 50 is not the gap, and the real gap points to osmotic diarrhoea.

Why is "90 mOsm/kg, indicating secretory diarrhoea" not the answer?

A value of 90 would be a miscalculation, and the real gap is not in the secretory range.

What concept does this question test?

It tests GI physiology: osmotic diarrhoea in Physiology. Here 2 times (30 plus 20) is 100, and 290 minus 100 gives 190 mOsm/kg. A gap above about 100 to 125 means unmeasured osmoles such as unabsorbed lactose, as in lactase deficiency, while a gap below about 50 suggests secretory diarrhoea.

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 GI physiology: osmotic diarrhoea?

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.