Acid-base: normal anion gap acidosis: 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: normal anion gap acidosis

Sample item

A 6-year-old with 2 days of watery diarrhoea has serum Na 138, K 3.0, Cl 112 and HCO3 14 mmol/L. What is the anion gap, and what is the likely cause of his acidosis?

A12 mmol/L, from accumulated lactic acidB12 mmol/L, from gastrointestinal bicarbonate lossC26 mmol/L, from accumulated ketoacidsD26 mmol/L, from retained phosphate and sulphate

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

The answer and the reasoning

B. 12 mmol/L, from gastrointestinal bicarbonate loss

Anion gap = Na - (Cl + HCO3) = 138 - (112 + 14) = 12 mmol/L, which is normal. Stool bicarbonate loss is replaced by chloride, giving hyperchloraemic (normal anion gap) acidosis. Lactic acid, ketoacids and renal failure raise the anion gap.

Option by option

A. 12 mmol/L, from accumulated lactic acid
The gap of 12 is normal, so unmeasured lactate is not accumulating.
B. 12 mmol/L, from gastrointestinal bicarbonate loss (correct)
Correct. A normal gap with high chloride indicates bicarbonate loss, as in diarrhoea.
C. 26 mmol/L, from accumulated ketoacids
The gap is not 26; 138 - 126 = 12, and ketoacids would raise it.
D. 26 mmol/L, from retained phosphate and sulphate
The gap is not 26 and renal retention of acids would raise the gap.

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 B, 12 mmol/L, from gastrointestinal bicarbonate loss. Anion gap = Na - (Cl + HCO3) = 138 - (112 + 14) = 12 mmol/L, which is normal.

Why is "12 mmol/L, from accumulated lactic acid" not the answer?

The gap of 12 is normal, so unmeasured lactate is not accumulating.

Why is "26 mmol/L, from accumulated ketoacids" not the answer?

The gap is not 26; 138 - 126 = 12, and ketoacids would raise it.

Why is "26 mmol/L, from retained phosphate and sulphate" not the answer?

The gap is not 26 and renal retention of acids would raise the gap.

What concept does this question test?

It tests Acid-base: normal anion gap acidosis in Physiology. Stool bicarbonate loss is replaced by chloride, giving hyperchloraemic (normal anion gap) acidosis. Lactic acid, ketoacids and renal failure raise the anion gap.

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: normal anion gap acidosis?

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.