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Hypertension — MRCGP AKT MCQ

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HardCardiologyHypertensionMRCGP AKT

A 24-year-old who frequently uses nitrous oxide recreationally reports new paraesthesia and unsteadiness. Examination suggests a sensory neuropathy, but full blood count is normal. Which initial biochemical test is recommended for the suspected cause-specific vitamin B12 deficiency?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Serum methylmalonic acid concentration

The best answer is “Serum methylmalonic acid concentration”. Nitrous oxide can inactivate vitamin B12 even when total or active circulating B12 appears normal. NICE therefore recommends serum methylmalonic acid or plasma homocysteine as the initial diagnostic test when recreational nitrous oxide is the suspected cause. Blood should be taken before replacement, but neurological disease warrants prompt treatment rather than waiting for results.

Reference: NICE NG239: Vitamin B12 deficiency in over 16s: https://www.nice.org.uk/guidance/ng239/chapter/recommendations