A 52-year-old male laborer from rural Uttar Pradesh presents with a 4-month history of progressive dermatitis on his face, neck, and hands (areas exposed to sunlight), chronic diarrhea, and increasing irritability and poor memory. His diet consists mainly of corn and legumes with minimal vegetables, eggs, or meat. On examination, he has a sharply demarcated, hyperpigmented, scaly dermatitis in a 'glove and stocking' distribution on sun-exposed areas. His mental status is mildly impaired. What is the most likely diagnosis?