Carbohydrate digestion: sucrase-isomaltase 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: sucrase-isomaltase deficiency

Sample item

A 2-year-old has watery diarrhoea and bloating after sweets and table sugar, but tolerates milk. Stool is negative for reducing substances until it is boiled with dilute acid, after which it becomes strongly positive. There is no hypoglycaemia or liver disease. Which enzyme is deficient?

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

The answer and the reasoning

C. Sucrase-isomaltase of the intestinal brush border

Sucrose is a non-reducing disaccharide, so undigested sucrose is detected only after acid hydrolysis releases glucose and fructose. Sucrase-isomaltase deficiency causes osmotic diarrhoea with sucrose and starch dextrins, with tolerance of lactose. Hereditary fructose intolerance would add hypoglycaemia and hepatic injury.

Option by option

  • A. Lactase of the intestinal brush border

    Lactase deficiency would cause symptoms with milk, which this child tolerates.

  • B. Pancreatic alpha-amylase

    Amylase deficiency is rare and would not produce a sucrose-specific stool picture.

  • C. Sucrase-isomaltase of the intestinal brush border (correct)

    Correct. Sucrase deficiency leaves sucrose undigested and non-reducing until hydrolysed.

  • D. Hepatic aldolase B

    Aldolase B deficiency causes hypoglycaemia, vomiting and liver failure, which are absent here.

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, Sucrase-isomaltase of the intestinal brush border. Sucrose is a non-reducing disaccharide, so undigested sucrose is detected only after acid hydrolysis releases glucose and fructose.

Why is "Lactase of the intestinal brush border" not the answer?

Lactase deficiency would cause symptoms with milk, which this child tolerates.

Why is "Pancreatic alpha-amylase" not the answer?

Amylase deficiency is rare and would not produce a sucrose-specific stool picture.

Why is "Hepatic aldolase B" not the answer?

Aldolase B deficiency causes hypoglycaemia, vomiting and liver failure, which are absent here.

What concept does this question test?

It tests Carbohydrate digestion: sucrase-isomaltase deficiency in Biochemistry. Sucrase-isomaltase deficiency causes osmotic diarrhoea with sucrose and starch dextrins, with tolerance of lactose. Hereditary fructose intolerance would add hypoglycaemia and hepatic injury.

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 Carbohydrate digestion: sucrase-isomaltase 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.