Clinical biochemistry: low anion gap and paraprotein: 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: low anion gap and paraprotein

Sample item

A 65-year-old with anaemia and back pain has a calculated anion gap of 2 mmol/L, a raised total protein, normal albumin and no history of drug use. What is the most likely cause of the low gap?

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

The answer and the reasoning

C. Retained cationic paraprotein from an IgG myeloma

A strongly cationic IgG paraprotein adds unmeasured cations, which lowers the calculated anion gap. A low gap with high protein and normal albumin should lead to protein electrophoresis. Albumin loss, lithium or bromide would not fit the stated findings.

Option by option

  • A. Bromide intoxication

    Bromide would be an unmeasured anion given by exposure, not suggested here.

  • B. Lithium toxicity

    No drug history is given to suggest lithium.

  • C. Retained cationic paraprotein from an IgG myeloma (correct)

    Correct. Unmeasured cationic paraprotein lowers the gap in myeloma.

  • D. Hypoalbuminaemia

    The albumin is normal in this patient.

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 C, Retained cationic paraprotein from an IgG myeloma. A strongly cationic IgG paraprotein adds unmeasured cations, which lowers the calculated anion gap.

Why is "Bromide intoxication" not the answer?

Bromide would be an unmeasured anion given by exposure, not suggested here.

Why is "Lithium toxicity" not the answer?

No drug history is given to suggest lithium.

Why is "Hypoalbuminaemia" not the answer?

The albumin is normal in this patient.

What concept does this question test?

It tests Clinical biochemistry: low anion gap and paraprotein in Biochemistry. A low gap with high protein and normal albumin should lead to protein electrophoresis. Albumin loss, lithium or bromide would not fit the stated findings.

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: low anion gap and paraprotein?

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.