A 38-year-old female from rural Maharashtra presents with a 6-month history of progressive weakness, paresthesias in both feet, and gait disturbance. On examination, she has loss of vibration sense and proprioception in the lower limbs, brisk reflexes, and a positive Romberg sign. She reports a predominantly cereal-based diet with minimal animal products. Serum vitamin B12 level is 180 pg/mL (normal >200). What is the most appropriate next step in management?