Acid-base: anion gap and urine anion gap: 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 · Acid-base: anion gap and urine anion gap

Sample item

A 6-year-old has had 3 days of watery diarrhoea. Serum sodium is 134, chloride 112 and bicarbonate 12 mmol/L. What are the serum anion gap and the expected urine anion gap?

ARaised (22 mmol/L), with a negative urine anion gapBRaised (22 mmol/L), with a positive urine anion gapCNormal (10 mmol/L), with a negative urine anion gapDNormal (10 mmol/L), with a positive urine anion gap

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

The answer and the reasoning

C. Normal (10 mmol/L), with a negative urine anion gap

Serum anion gap = Na minus (Cl + HCO3) = 134 - (112 + 12) = 10 mmol/L, which is normal, so this is a hyperchloraemic non-anion-gap acidosis. In gastrointestinal bicarbonate loss the kidney responds by excreting ammonium chloride, so urinary chloride exceeds sodium plus potassium and the urine anion gap is negative. A positive gap suggests renal tubular acidosis because ammonium excretion is impaired.

Option by option

A. Raised (22 mmol/L), with a negative urine anion gap
The arithmetic gives 10, not 22, and a high gap would indicate unmeasured anions such as lactate.
B. Raised (22 mmol/L), with a positive urine anion gap
Both the gap calculation and the urine gap are wrong for diarrhoeal bicarbonate loss.
C. Normal (10 mmol/L), with a negative urine anion gap (correct)
Correct. The gap is 134 - 124 = 10 and the urine anion gap is negative because of high ammonium excretion.
D. Normal (10 mmol/L), with a positive urine anion gap
A positive urine anion gap points to distal renal tubular acidosis, not diarrhoeal loss.

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 C, Normal (10 mmol/L), with a negative urine anion gap. Serum anion gap = Na minus (Cl + HCO3) = 134 - (112 + 12) = 10 mmol/L, which is normal, so this is a hyperchloraemic non-anion-gap acidosis.

Why is "Raised (22 mmol/L), with a negative urine anion gap" not the answer?

The arithmetic gives 10, not 22, and a high gap would indicate unmeasured anions such as lactate.

Why is "Raised (22 mmol/L), with a positive urine anion gap" not the answer?

Both the gap calculation and the urine gap are wrong for diarrhoeal bicarbonate loss.

Why is "Normal (10 mmol/L), with a positive urine anion gap" not the answer?

A positive urine anion gap points to distal renal tubular acidosis, not diarrhoeal loss.

What concept does this question test?

It tests Acid-base: anion gap and urine anion gap in Biochemistry. In gastrointestinal bicarbonate loss the kidney responds by excreting ammonium chloride, so urinary chloride exceeds sodium plus potassium and the urine anion gap is negative. A positive gap suggests renal tubular acidosis because ammonium excretion is impaired.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Acid-base: anion gap and urine anion gap?

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.