A 42-year-old woman from rural Maharashtra presents with a 6-month history of progressive weakness, paresthesias in her feet, and difficulty walking. On examination, she has a stocking-glove distribution of sensory loss, absent ankle reflexes, and positive Romberg's sign. Her diet consists primarily of polished rice with minimal protein intake. Blood investigations show macrocytic anemia (Hb 8.2 g/dL, MCV 108 fL) and elevated methylmalonic acid levels. What is the most likely diagnosis?