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Vitamin B12 Deficiency (Crohn's) — UKMLA MCQ

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HardHaematologyVitamin B12 Deficiency (Crohn's)UKMLAMRCGP AKTPARAMRCP Part 1

A person with terminal-ileal Crohn disease has macrocytosis, glossitis and paraesthesia. Serum vitamin B12 is indeterminate. Which additional blood marker is most useful for supporting functional vitamin B12 deficiency?

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

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Correct answer: B — Serum methylmalonic acid

B is correct. When the initial vitamin B12 result is indeterminate and clinical suspicion persists, NICE recommends considering serum methylmalonic acid; it rises when intracellular vitamin B12-dependent metabolism is impaired. Interpretation must account for renal impairment, which can also raise MMA. A evaluates iron stores. C supports a haemolysis assessment. D is used in selected copper disorders, and E does not diagnose vitamin B12 deficiency. Neurological symptoms require prompt treatment rather than waiting for every confirmatory result, and terminal-ileal disease is a strong malabsorption risk factor.

Reference: NICE NG239 vitamin B12 deficiency: https://www.nice.org.uk/guidance/ng239/chapter/Recommendations GMC PA registration assessment content map: https://www.gmc-uk.org/education/standards-guidance-and-curricula/standards-and-outcomes/pa-and-aa-prequalification-education-framework/pa-registration-assessment-content-map