A 38-year-old female from rural Maharashtra presents with a 3-month history of progressive numbness and tingling in her feet, followed by weakness in her lower limbs. On examination, she has diminished vibration and proprioception sense in both lower limbs, and a positive Romberg sign. Her diet consists mainly of polished rice and legumes with minimal animal products. Serum methylmalonic acid and homocysteine levels are elevated. What is the most appropriate next step in management?