A 52-year-old alcoholic notices a skin rash on his chest, and also has symptoms of diarrhea and abdominal pain. On examination, he has a scaly and pigmented rash on the sun-exposed areas of his skin, the abdomen is soft, and his short-term memory is impaired. He has dermatitis, diarrhea, and dementia syndrome.For the patient with vitamin deficiency or excess, select the most likely diagnosis

Diarrhea, dementia, and dermatitis are the classic triad for pellagra (niacin deficiency). The diagnosis is based on clinical suspicion and response to therapy, and can be confirmed by demonstrating low levels of the urinary metabolites 2-methylnicotinamide and 2-pyridone. Small doses of niacin (10 mg/day) with adequate dietary tryptophan will cure pellagra secondary to nutritional deficiency.