Carbohydrate metabolism: polyol pathway: 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 metabolism: polyol pathway

Sample item

In a patient with poorly controlled diabetes, which mechanism best explains osmotic swelling of lens fibres leading to cataract?

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

The answer and the reasoning

B. Intracellular sorbitol accumulates because aldose reductase makes it faster than sorbitol dehydrogenase removes it

In hyperglycaemia, aldose reductase converts excess glucose to sorbitol using NADPH. Lens, retina, Schwann cells and renal papilla have little sorbitol dehydrogenase, so the polar sorbitol is trapped and draws in water. Osmotic swelling damages the lens fibres and is one mechanism of diabetic cataract and neuropathy.

Option by option

  • A. Excess intracellular fructose is phosphorylated and traps phosphate

    Phosphate trapping by fructokinase describes the hepatic effect of fructose loading, not lens swelling.

  • B. Intracellular sorbitol accumulates because aldose reductase makes it faster than sorbitol dehydrogenase removes it (correct)

    Trapped sorbitol creates an osmotic load in tissues with low sorbitol dehydrogenase, swelling the lens.

  • C. Lactate accumulates through increased anaerobic glycolysis in the lens

    Lens metabolism is largely anaerobic, but the lactate produced diffuses out and is not the cause of swelling.

  • D. Galactitol accumulates from galactokinase deficiency

    Galactitol formation occurs in galactosemia, which is not the situation in diabetes.

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 B, Intracellular sorbitol accumulates because aldose reductase makes it faster than sorbitol dehydrogenase removes it. In hyperglycaemia, aldose reductase converts excess glucose to sorbitol using NADPH.

Why is "Excess intracellular fructose is phosphorylated and traps phosphate" not the answer?

Phosphate trapping by fructokinase describes the hepatic effect of fructose loading, not lens swelling.

Why is "Lactate accumulates through increased anaerobic glycolysis in the lens" not the answer?

Lens metabolism is largely anaerobic, but the lactate produced diffuses out and is not the cause of swelling.

Why is "Galactitol accumulates from galactokinase deficiency" not the answer?

Galactitol formation occurs in galactosemia, which is not the situation in diabetes.

What concept does this question test?

It tests Carbohydrate metabolism: polyol pathway in Biochemistry. Lens, retina, Schwann cells and renal papilla have little sorbitol dehydrogenase, so the polar sorbitol is trapped and draws in water. Osmotic swelling damages the lens fibres and is one mechanism of diabetic cataract and neuropathy.

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 metabolism: polyol pathway?

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.