A 38-year-old woman from rural Bihar presents with a 6-month history of progressive weakness, paresthesias in both feet, and unsteady gait. On examination, she has loss of vibration and proprioception in lower limbs, positive Romberg sign, and hyporeflexia. Her hemoglobin is 9.2 g/dL with macrocytic indices (MCV 104 fL). Serum B12 level is 180 pg/mL (normal >200). She reports a predominantly vegetarian diet with rare egg consumption. What is the most appropriate next step in management?