A 48-year-old woman on long-term omeprazole (40 mg daily for 5 years) for GERD presents with fatigue, paresthesias, and macrocytic anemia (Hb 8.2 g/dL, MCV 108 fL). Serum B12 is 180 pg/mL (normal 200–900). Which investigation is most appropriate to confirm the etiology of her B12 deficiency?

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