Carbohydrate metabolism: uronic acid 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: uronic acid pathway

Sample item

A healthy 35-year-old man of Ashkenazi Jewish descent has repeatedly positive urinary Benedict tests with a negative glucose oxidase result. His blood glucose is normal and the orcinol test identifies a pentose. Which enzyme is deficient?

AFructokinaseBL-xylulose reductaseCAldose reductaseDUDP-glucuronyl transferase

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

The answer and the reasoning

B. L-xylulose reductase

Essential pentosuria is a benign defect of the uronic acid pathway in which L-xylulose cannot be reduced to xylitol, so it is excreted in urine. It is important because it gives a false impression of diabetes mellitus on a reducing sugar test, but it causes no illness and needs no treatment.

Option by option

A. Fructokinase
Fructokinase deficiency gives fructosuria, in which the sugar is a ketohexose rather than a pentose.
B. L-xylulose reductase (correct)
L-xylulose accumulates and is excreted harmlessly, producing a reducing but glucose oxidase negative urine.
C. Aldose reductase
Aldose reductase is a polyol pathway enzyme and its absence does not cause pentosuria.
D. UDP-glucuronyl transferase
UDP-glucuronyl transferase deficiency causes unconjugated hyperbilirubinaemia such as Crigler-Najjar syndrome.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, L-xylulose reductase. Essential pentosuria is a benign defect of the uronic acid pathway in which L-xylulose cannot be reduced to xylitol, so it is excreted in urine.

Why is "Fructokinase" not the answer?

Fructokinase deficiency gives fructosuria, in which the sugar is a ketohexose rather than a pentose.

Why is "Aldose reductase" not the answer?

Aldose reductase is a polyol pathway enzyme and its absence does not cause pentosuria.

Why is "UDP-glucuronyl transferase" not the answer?

UDP-glucuronyl transferase deficiency causes unconjugated hyperbilirubinaemia such as Crigler-Najjar syndrome.

What concept does this question test?

It tests Carbohydrate metabolism: uronic acid pathway in Biochemistry. It is important because it gives a false impression of diabetes mellitus on a reducing sugar test, but it causes no illness 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: uronic acid 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.