A 52-year-old woman from rural Maharashtra presents with progressive fatigue and dyspnea on exertion for 6 months. She reports a diet poor in vegetables and meat. On examination, she has glossitis and a sensory ataxia affecting her lower limbs. Her hemoglobin is 8.2 g/dL with a mean corpuscular volume (MCV) of 104 fL. Peripheral blood smear shows hypersegmented neutrophils. Serum vitamin B₁₂ level is 180 pg/mL (normal: 200–900). Serum folate is 3.2 ng/mL (normal: >5.4). What is the most appropriate next step in the workup?