Carbohydrate metabolism: galactosemia: 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: galactosemia

Sample item

A 10-day-old breastfed neonate has jaundice, vomiting and poor feeding, and develops Escherichia coli sepsis. Slit-lamp examination shows bilateral oil-drop cataracts. Urine is positive for reducing substances but negative on a glucose oxidase strip. Which metabolite is chiefly responsible for the lens opacity?

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

The answer and the reasoning

B. Galactitol formed by aldose reductase

In classic galactosemia, galactose-1-phosphate uridyltransferase is deficient, so galactose accumulates and is reduced by aldose reductase to galactitol. Galactitol cannot leave the lens, draws in water and causes osmotic swelling and cataract. Galactose-1-phosphate is the main toxin in liver, kidney and brain, and the neonate must stop lactose-containing feeds at once.

Option by option

  • A. Galactose-1-phosphate accumulating in the lens

    Galactose-1-phosphate mainly injures liver, kidney and brain, not the lens, and it is not the polyol that traps water.

  • B. Galactitol formed by aldose reductase (correct)

    Correct. Galactitol is trapped in the lens and acts osmotically to produce cataract.

  • C. Sorbitol formed by aldose reductase from glucose

    Sorbitol is the polyol of glucose and underlies diabetic cataract; here the sugar accumulating is galactose.

  • D. Fructose-1-phosphate trapped in lens fibres

    Fructose-1-phosphate accumulates in hereditary fructose intolerance, which does not cause neonatal cataract.

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, Galactitol formed by aldose reductase. In classic galactosemia, galactose-1-phosphate uridyltransferase is deficient, so galactose accumulates and is reduced by aldose reductase to galactitol.

Why is "Galactose-1-phosphate accumulating in the lens" not the answer?

Galactose-1-phosphate mainly injures liver, kidney and brain, not the lens, and it is not the polyol that traps water.

Why is "Sorbitol formed by aldose reductase from glucose" not the answer?

Sorbitol is the polyol of glucose and underlies diabetic cataract; here the sugar accumulating is galactose.

Why is "Fructose-1-phosphate trapped in lens fibres" not the answer?

Fructose-1-phosphate accumulates in hereditary fructose intolerance, which does not cause neonatal cataract.

What concept does this question test?

It tests Carbohydrate metabolism: galactosemia in Biochemistry. Galactitol cannot leave the lens, draws in water and causes osmotic swelling and cataract. Galactose-1-phosphate is the main toxin in liver, kidney and brain, and the neonate must stop lactose-containing feeds at once.

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: galactosemia?

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.