Carbohydrate metabolism: essential fructosuria: 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: essential fructosuria

Sample item

A healthy 6-year-old child has a routine urine test positive for reducing substances but negative on a glucose oxidase strip, especially after fruit juice. There is no vomiting, hypoglycaemia or hepatomegaly. Which enzyme deficiency explains this benign finding?

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

The answer and the reasoning

C. Fructokinase

Fructokinase normally phosphorylates fructose to fructose-1-phosphate in the liver. When it is absent, fructose simply accumulates and is excreted in urine, with no toxic phosphorylated intermediate, so the condition is asymptomatic and needs no treatment.

Option by option

  • A. Fructose-1,6-bisphosphatase

    Fructose-1,6-bisphosphatase deficiency causes fasting hypoglycaemia and lactic acidosis from impaired gluconeogenesis.

  • B. Galactose-1-phosphate uridyltransferase

    Uridyltransferase deficiency is galactosemia, which makes a neonate ill with milk feeding.

  • C. Fructokinase (correct)

    Absent fructokinase leaves fructose unmetabolised and excreted, with no trapped phosphate, so the child is well.

  • D. Aldolase B

    Aldolase B deficiency traps fructose-1-phosphate and produces vomiting, hypoglycaemia and liver injury.

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, Fructokinase. Fructokinase normally phosphorylates fructose to fructose-1-phosphate in the liver.

Why is "Fructose-1,6-bisphosphatase" not the answer?

Fructose-1,6-bisphosphatase deficiency causes fasting hypoglycaemia and lactic acidosis from impaired gluconeogenesis.

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

Uridyltransferase deficiency is galactosemia, which makes a neonate ill with milk feeding.

Why is "Aldolase B" not the answer?

Aldolase B deficiency traps fructose-1-phosphate and produces vomiting, hypoglycaemia and liver injury.

What concept does this question test?

It tests Carbohydrate metabolism: essential fructosuria in Biochemistry. When it is absent, fructose simply accumulates and is excreted in urine, with no toxic phosphorylated intermediate, so the condition is asymptomatic and needs no treatment.

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: essential fructosuria?

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.