Carbohydrate digestion: lactase deficiency: 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.

Try it

Biochemistry · Carbohydrate digestion: lactase deficiency

Sample item

A 24-year-old woman has bloating, cramps and watery diarrhoea within an hour of drinking milk, relieved by avoiding dairy. Which stool finding is expected during an attack?

ALarge amounts of undigested muscle fibresBAcidic stool pH below 5.5 with reducing substancesCBlood and mucus with pus cellsDAlkaline stool pH with steatorrhoea

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

The answer and the reasoning

B. Acidic stool pH below 5.5 with reducing substances

In adult-type lactase non-persistence, unabsorbed lactose is osmotically active and is fermented by colonic bacteria to short-chain fatty acids, lactic acid, hydrogen and carbon dioxide. The stool becomes acidic and contains reducing sugar. A hydrogen breath test confirms the diagnosis.

Option by option

A. Large amounts of undigested muscle fibres
Undigested muscle fibres point to pancreatic proteolytic failure.
B. Acidic stool pH below 5.5 with reducing substances (correct)
Bacterial fermentation of unabsorbed lactose acidifies the stool, and lactose is a reducing sugar.
C. Blood and mucus with pus cells
Blood, mucus and pus indicate invasive infection or inflammatory bowel disease.
D. Alkaline stool pH with steatorrhoea
Alkaline stool with fat suggests pancreatic or biliary steatorrhoea, not carbohydrate malabsorption.

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 B, Acidic stool pH below 5.5 with reducing substances. In adult-type lactase non-persistence, unabsorbed lactose is osmotically active and is fermented by colonic bacteria to short-chain fatty acids, lactic acid, hydrogen and carbon dioxide.

Why is "Large amounts of undigested muscle fibres" not the answer?

Undigested muscle fibres point to pancreatic proteolytic failure.

Why is "Blood and mucus with pus cells" not the answer?

Blood, mucus and pus indicate invasive infection or inflammatory bowel disease.

Why is "Alkaline stool pH with steatorrhoea" not the answer?

Alkaline stool with fat suggests pancreatic or biliary steatorrhoea, not carbohydrate malabsorption.

What concept does this question test?

It tests Carbohydrate digestion: lactase deficiency in Biochemistry. The stool becomes acidic and contains reducing sugar. A hydrogen breath test confirms the diagnosis.

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 Carbohydrate digestion: lactase deficiency?

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.