Acid-base: urinary anion gap: practice question

A Physiology 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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Physiology · Acid-base: urinary anion gap

Sample item

After 5 days of watery diarrhoea, a man has pH 7.28, serum bicarbonate 14 mmol/L, Na 138, Cl 114 and K 3.0 mmol/L. Urine Na 40, K 30 and Cl 80 mmol/L. What does the urinary anion gap show?

AIt cannot be interpreted when the serum potassium is lowBIt is positive (+10), showing a distal renal acidification defectCIt is zero, showing equal cation and anion excretionDIt is negative (-10), showing high urinary ammonium and normal renal acidification

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

The answer and the reasoning

D. It is negative (-10), showing high urinary ammonium and normal renal acidification

The serum anion gap is 138 - (114 + 14) = 10, which is normal. Urinary anion gap = Na + K - Cl = 40 + 30 - 80 = -10. A negative gap means that unmeasured ammonium chloride is being excreted in large amounts, so renal acidification is intact and the bicarbonate loss is extrarenal. A positive gap in a similar acidosis suggests renal tubular acidosis.

Option by option

A. It cannot be interpreted when the serum potassium is low
The urinary anion gap remains useful in hypokalaemia, which is common with diarrhoea.
B. It is positive (+10), showing a distal renal acidification defect
A positive gap would be seen in distal renal tubular acidosis, not here.
C. It is zero, showing equal cation and anion excretion
The calculated value is -10, not zero.
D. It is negative (-10), showing high urinary ammonium and normal renal acidification (correct)
Correct. Ammonium excretion is high, pointing to gut bicarbonate 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 Physiology question

What is the answer to this Physiology question?

The best answer is D, It is negative (-10), showing high urinary ammonium and normal renal acidification. The serum anion gap is 138 - (114 + 14) = 10, which is normal.

Why is "It cannot be interpreted when the serum potassium is low" not the answer?

The urinary anion gap remains useful in hypokalaemia, which is common with diarrhoea.

Why is "It is positive (+10), showing a distal renal acidification defect" not the answer?

A positive gap would be seen in distal renal tubular acidosis, not here.

Why is "It is zero, showing equal cation and anion excretion" not the answer?

The calculated value is -10, not zero.

What concept does this question test?

It tests Acid-base: urinary anion gap in Physiology. Urinary anion gap = Na + K - Cl = 40 + 30 - 80 = -10. A negative gap means that unmeasured ammonium chloride is being excreted in large amounts, so renal acidification is intact and the bicarbonate loss is extrarenal. A positive gap in a similar acidosis suggests renal tubular acidosis.

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: urinary anion gap?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.