A 38-year-old man from northern India presents with a 4-month history of dermatitis on sun-exposed areas (face, neck, hands), chronic diarrhea, and mood disturbances including irritability and poor concentration. He works as a laborer and his diet consists mainly of corn and legumes. On examination, he has a well-demarcated erythematous rash with scaling on the dorsal surfaces of his hands and face. His serum albumin is 2.8 g/dL. What is the most likely vitamin deficiency?