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.

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Biochemistry · Carbohydrate digestion: lactase deficiency

Sample item

A 9-month-old infant recovering from rotavirus gastroenteritis develops explosive watery stools, perianal excoriation and abdominal distension after restarting formula feeds. The stool pH is 5.0 and stool reducing substances are positive. What is the most likely underlying mechanism?

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

The answer and the reasoning

C. Reduced brush-border lactase causing unabsorbed lactose to ferment into organic acids

Secondary lactase deficiency follows villous injury, as lactase is the most superficial and vulnerable brush-border disaccharidase. Lactose is a reducing sugar, so unabsorbed lactose gives positive reducing substances and bacterial fermentation produces short-chain acids and gas, lowering stool pH below 5.5. Lactose-free or lactose-reduced feeds usually settle symptoms.

Option by option

  • A. Failure of GLUT5 transport causing fructose malabsorption

    Fructose malabsorption through GLUT5 occurs with fruit or juice load, and fructose is not the sugar in standard formula.

  • B. Reduced sucrase-isomaltase causing sucrose malabsorption

    Sucrose is a non-reducing sugar, so stool reducing substances would be negative without prior hydrolysis.

  • C. Reduced brush-border lactase causing unabsorbed lactose to ferment into organic acids (correct)

    Correct. Unabsorbed lactose is osmotically active, is a reducing sugar and ferments to acids.

  • D. Reduced pancreatic amylase causing starch malabsorption

    Amylase deficiency is rare in infants and starch is not a reducing sugar, so the stool test would be negative.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Reduced brush-border lactase causing unabsorbed lactose to ferment into organic acids. Secondary lactase deficiency follows villous injury, as lactase is the most superficial and vulnerable brush-border disaccharidase.

Why is "Failure of GLUT5 transport causing fructose malabsorption" not the answer?

Fructose malabsorption through GLUT5 occurs with fruit or juice load, and fructose is not the sugar in standard formula.

Why is "Reduced sucrase-isomaltase causing sucrose malabsorption" not the answer?

Sucrose is a non-reducing sugar, so stool reducing substances would be negative without prior hydrolysis.

Why is "Reduced pancreatic amylase causing starch malabsorption" not the answer?

Amylase deficiency is rare in infants and starch is not a reducing sugar, so the stool test would be negative.

What concept does this question test?

It tests Carbohydrate digestion: lactase deficiency in Biochemistry. Lactose is a reducing sugar, so unabsorbed lactose gives positive reducing substances and bacterial fermentation produces short-chain acids and gas, lowering stool pH below 5.5. Lactose-free or lactose-reduced feeds usually settle symptoms.

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.