Nephrology: normal anion gap acidosis: practice question

A General Medicine 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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General Medicine · Nephrology: normal anion gap acidosis

Sample item

A patient with severe diarrhoea has pH 7.28, bicarbonate 14 mmol/L, chloride 115 mmol/L, sodium 138 mmol/L. What is the urine anion gap likely to show?

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

The answer and the reasoning

B. Negative, due to increased renal ammonium excretion

In diarrhoeal acidosis, the kidney responds appropriately by excreting ammonium, so the urine anion gap (Na plus K minus Cl) is negative. A positive urine anion gap in a normal-gap acidosis indicates impaired ammonium excretion, as in distal RTA. This helps separate gastrointestinal from renal causes.

Option by option

  • A. Positive, due to distal RTA

    A positive gap would suggest renal cause.

  • B. Negative, due to increased renal ammonium excretion (correct)

    Correct. A negative gap means appropriate renal ammonium excretion.

  • C. Zero in all cases

    The gap is not always zero.

  • D. Unhelpful and always elevated

    It is helpful and is not always raised.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is B, Negative, due to increased renal ammonium excretion. In diarrhoeal acidosis, the kidney responds appropriately by excreting ammonium, so the urine anion gap (Na plus K minus Cl) is negative.

Why is "Positive, due to distal RTA" not the answer?

A positive gap would suggest renal cause.

Why is "Zero in all cases" not the answer?

The gap is not always zero.

Why is "Unhelpful and always elevated" not the answer?

It is helpful and is not always raised.

What concept does this question test?

It tests Nephrology: normal anion gap acidosis in General Medicine. A positive urine anion gap in a normal-gap acidosis indicates impaired ammonium excretion, as in distal RTA. This helps separate gastrointestinal from renal causes.

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 Nephrology: normal anion gap acidosis?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.