Acid-base: 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.

Try it

Physiology · Acid-base: anion gap

Sample item

A 24-year-old with type 1 diabetes has vomiting and deep, rapid breathing. Arterial pH is 7.18 and bicarbonate is 8 mmol/L. Serum sodium is 138, and chloride 96 mmol/L. Which process best explains the acidosis?

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

The answer and the reasoning

C. Accumulation of unmeasured acids (ketoacids)

The anion gap is sodium minus the sum of chloride and bicarbonate, here 138 minus 104, which is 34 mmol/L and well above the normal range. A raised gap means unmeasured anions such as ketoacids have replaced bicarbonate. The deep breathing is respiratory compensation that lowers PaCO2.

Option by option

  • A. Distal renal tubular acidosis

    Distal renal tubular acidosis also causes a normal anion gap acidosis with chloride retention.

  • B. Large-volume saline infusion

    Saline infusion causes hyperchloraemic acidosis with a normal anion gap.

  • C. Accumulation of unmeasured acids (ketoacids) (correct)

    Correct. A high gap with insulin deficiency indicates ketoacid accumulation as in diabetic ketoacidosis.

  • D. Bicarbonate loss through diarrhoea

    Bicarbonate loss in diarrhoea is replaced by chloride and gives a normal anion gap acidosis.

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.

More Physiology questions

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Accumulation of unmeasured acids (ketoacids). The anion gap is sodium minus the sum of chloride and bicarbonate, here 138 minus 104, which is 34 mmol/L and well above the normal range.

Why is "Distal renal tubular acidosis" not the answer?

Distal renal tubular acidosis also causes a normal anion gap acidosis with chloride retention.

Why is "Large-volume saline infusion" not the answer?

Saline infusion causes hyperchloraemic acidosis with a normal anion gap.

Why is "Bicarbonate loss through diarrhoea" not the answer?

Bicarbonate loss in diarrhoea is replaced by chloride and gives a normal anion gap acidosis.

What concept does this question test?

It tests Acid-base: anion gap in Physiology. A raised gap means unmeasured anions such as ketoacids have replaced bicarbonate. The deep breathing is respiratory compensation that lowers PaCO2.

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?

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.