Carbohydrate metabolism: galactokinase 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 metabolism: galactokinase deficiency

Sample item

A 3-month-old breastfed infant has bilateral lens opacities. There is no jaundice, hepatomegaly, vomiting or sepsis, and urine is positive for a reducing sugar. Galactose-1-phosphate uridyltransferase activity is normal. Which enzyme is deficient?

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

The answer and the reasoning

C. Galactokinase

Galactokinase deficiency blocks phosphorylation of galactose, so galactose accumulates and is reduced by aldose reductase to galactitol, which causes osmotic cataract. Because galactose-1-phosphate does not build up, liver, kidney and brain are spared, unlike classic galactosaemia.

Option by option

  • A. UDP-galactose 4-epimerase

    Epimerase deficiency, in its severe form, resembles transferase deficiency with systemic features, and the mild form is usually silent.

  • B. Galactose-1-phosphate uridyltransferase

    Transferase deficiency causes classic galactosaemia with jaundice, hepatomegaly and E. coli sepsis, and its activity is stated to be normal.

  • C. Galactokinase (correct)

    Correct. Galactokinase deficiency causes isolated cataract from galactitol without hepatic toxicity.

  • D. Aldose reductase

    Aldose reductase is the enzyme that makes galactitol, so a deficiency would protect against, not cause, 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 C, Galactokinase. Galactokinase deficiency blocks phosphorylation of galactose, so galactose accumulates and is reduced by aldose reductase to galactitol, which causes osmotic cataract.

Why is "UDP-galactose 4-epimerase" not the answer?

Epimerase deficiency, in its severe form, resembles transferase deficiency with systemic features, and the mild form is usually silent.

Why is "Galactose-1-phosphate uridyltransferase" not the answer?

Transferase deficiency causes classic galactosaemia with jaundice, hepatomegaly and E. coli sepsis, and its activity is stated to be normal.

Why is "Aldose reductase" not the answer?

Aldose reductase is the enzyme that makes galactitol, so a deficiency would protect against, not cause, cataract.

What concept does this question test?

It tests Carbohydrate metabolism: galactokinase deficiency in Biochemistry. Because galactose-1-phosphate does not build up, liver, kidney and brain are spared, unlike classic galactosaemia.

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: galactokinase 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.