Gastrointestinal: carbohydrate malabsorption: 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 · Gastrointestinal: carbohydrate malabsorption

Sample item

An infant has explosive watery stools with acidic pH after milk feeds, and the stool contains reducing substances. Which mechanism explains the diarrhoea?

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

The answer and the reasoning

C. Unabsorbed lactose holds water and is fermented to gas and acids

Lactase deficiency leaves lactose undigested in the lumen, where it exerts osmotic pull and is fermented by colonic bacteria to gas and short chain acids. This lowers stool pH and gives a positive hydrogen breath test. It is an osmotic diarrhoea that stops on fasting.

Option by option

  • A. Invasion of the mucosa by bacteria

    Invasive infection causes inflammatory diarrhoea with blood and leucocytes.

  • B. Fat malabsorption from bile salt deficiency

    Fat malabsorption causes greasy steatorrhoea, not acidic stool with reducing sugars.

  • C. Unabsorbed lactose holds water and is fermented to gas and acids (correct)

    Correct. Osmotic retention of water plus bacterial fermentation explains acidic watery stools.

  • D. Increased chloride secretion by crypt cells

    Chloride hypersecretion causes secretory diarrhoea such as cholera, which continues despite fasting and has neutral stool.

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 C, Unabsorbed lactose holds water and is fermented to gas and acids. Lactase deficiency leaves lactose undigested in the lumen, where it exerts osmotic pull and is fermented by colonic bacteria to gas and short chain acids.

Why is "Invasion of the mucosa by bacteria" not the answer?

Invasive infection causes inflammatory diarrhoea with blood and leucocytes.

Why is "Fat malabsorption from bile salt deficiency" not the answer?

Fat malabsorption causes greasy steatorrhoea, not acidic stool with reducing sugars.

Why is "Increased chloride secretion by crypt cells" not the answer?

Chloride hypersecretion causes secretory diarrhoea such as cholera, which continues despite fasting and has neutral stool.

What concept does this question test?

It tests Gastrointestinal: carbohydrate malabsorption in Physiology. This lowers stool pH and gives a positive hydrogen breath test. It is an osmotic diarrhoea that stops on fasting.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Gastrointestinal: carbohydrate malabsorption?

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.