Clinical biochemistry: urine ketone testing: 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 · Clinical biochemistry: urine ketone testing

Sample item

A 16-year-old with type 1 diabetes arrives in severe acidosis. Capillary blood beta-hydroxybutyrate is 6 mmol/L, but the urine nitroprusside test shows only a trace of ketones. What explains this?

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

The answer and the reasoning

C. The nitroprusside reaction detects acetoacetate and not beta-hydroxybutyrate

In ketoacidosis the high NADH/NAD+ ratio favours beta-hydroxybutyrate over acetoacetate, so the dominant ketone is the one the nitroprusside reagent does not detect; it reacts with acetoacetate and, much more weakly, acetone. Urine tests can therefore underestimate the severity early in treatment, and may paradoxically strengthen as beta-hydroxybutyrate is oxidised to acetoacetate. Blood beta-hydroxybutyrate is the preferred measure.

Option by option

  • A. Ketone bodies are not excreted in urine in type 1 diabetes

    Ketone bodies are freely excreted in urine in diabetic ketoacidosis.

  • B. The patient has alcoholic rather than diabetic ketoacidosis

    The picture fits diabetic ketoacidosis in a known type 1 patient, and the discordance is explained by chemistry.

  • C. The nitroprusside reaction detects acetoacetate and not beta-hydroxybutyrate (correct)

    Correct. The dominant ketone is not detected by the nitroprusside reaction.

  • D. The nitroprusside reaction detects only acetone

    The reagent reacts mainly with acetoacetate, and acetone only weakly.

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, The nitroprusside reaction detects acetoacetate and not beta-hydroxybutyrate. In ketoacidosis the high NADH/NAD+ ratio favours beta-hydroxybutyrate over acetoacetate, so the dominant ketone is the one the nitroprusside reagent does not detect; it reacts with acetoacetate and, much more weakly, acetone.

Why is "Ketone bodies are not excreted in urine in type 1 diabetes" not the answer?

Ketone bodies are freely excreted in urine in diabetic ketoacidosis.

Why is "The patient has alcoholic rather than diabetic ketoacidosis" not the answer?

The picture fits diabetic ketoacidosis in a known type 1 patient, and the discordance is explained by chemistry.

Why is "The nitroprusside reaction detects only acetone" not the answer?

The reagent reacts mainly with acetoacetate, and acetone only weakly.

What concept does this question test?

It tests Clinical biochemistry: urine ketone testing in Biochemistry. Urine tests can therefore underestimate the severity early in treatment, and may paradoxically strengthen as beta-hydroxybutyrate is oxidised to acetoacetate. Blood beta-hydroxybutyrate is the preferred measure.

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 Clinical biochemistry: urine ketone testing?

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.