A 58-year-old Indian woman presents with progressive fatigue, dyspnea on exertion, and paresthesias in her feet for 3 months. She is a strict vegetarian who avoids dairy products. On examination, she has glossitis, mild jaundice, and diminished vibration sense in the lower limbs. Her hemoglobin is 7.2 g/dL, MCV is 112 fL, and reticulocyte count is 0.8%. Peripheral blood smear shows hypersegmented neutrophils and macro-ovalocytes. Serum vitamin B12 level is 180 pg/mL (normal: 200–900 pg/mL). What is the most likely diagnosis?