A 42-year-old man from rural Bihar presents with a 3-month history of progressive weakness, numbness in his feet, and difficulty walking. On examination, he has diminished vibration and proprioception sense in the lower limbs, absent ankle reflexes, and a positive Romberg sign. His diet consists mainly of polished rice with minimal vegetables or legumes. Blood glucose is 98 mg/dL. Serum B12 and folate levels are normal. Which vitamin deficiency is most likely responsible for his neurological findings?