Clinical biochemistry: 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 · Clinical biochemistry: anion gap

Sample item

A 55-year-old man in septic shock has serum sodium 138 mmol/L, chloride 96 mmol/L and bicarbonate 10 mmol/L, with a pH of 7.18. What is the anion gap and what does it indicate?

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

The answer and the reasoning

B. 32 mmol/L, a high anion gap acidosis due to unmeasured anions such as lactate

Anion gap equals sodium minus the sum of chloride and bicarbonate: 138 - (96 + 10) = 32 mmol/L, well above the normal 8 to 12. A raised gap means acid has been added with an unmeasured anion, such as lactate in shock, ketoacids, uraemic anions or toxic alcohol metabolites. Normal gap (hyperchloraemic) acidosis arises from bicarbonate loss in diarrhoea or renal tubular acidosis.

Option by option

  • A. 28 mmol/L, a normal anion gap acidosis due to renal tubular acidosis

    A gap of 28 is a miscalculation, and renal tubular acidosis has a normal gap.

  • B. 32 mmol/L, a high anion gap acidosis due to unmeasured anions such as lactate (correct)

    Correct. The calculated gap of 32 mmol/L reflects accumulation of unmeasured lactate.

  • C. 12 mmol/L, a normal anion gap acidosis due to bicarbonate loss

    A gap of 12 would need different electrolytes, and the arithmetic here gives 32.

  • D. 44 mmol/L, a high anion gap acidosis due to chloride retention

    The arithmetic is wrong, and chloride retention produces a normal-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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, 32 mmol/L, a high anion gap acidosis due to unmeasured anions such as lactate. Anion gap equals sodium minus the sum of chloride and bicarbonate: 138 - (96 + 10) = 32 mmol/L, well above the normal 8 to 12.

Why is "28 mmol/L, a normal anion gap acidosis due to renal tubular acidosis" not the answer?

A gap of 28 is a miscalculation, and renal tubular acidosis has a normal gap.

Why is "12 mmol/L, a normal anion gap acidosis due to bicarbonate loss" not the answer?

A gap of 12 would need different electrolytes, and the arithmetic here gives 32.

Why is "44 mmol/L, a high anion gap acidosis due to chloride retention" not the answer?

The arithmetic is wrong, and chloride retention produces a normal-gap acidosis.

What concept does this question test?

It tests Clinical biochemistry: anion gap in Biochemistry. A raised gap means acid has been added with an unmeasured anion, such as lactate in shock, ketoacids, uraemic anions or toxic alcohol metabolites. Normal gap (hyperchloraemic) acidosis arises from bicarbonate loss in diarrhoea or 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 Clinical biochemistry: 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.