Vitamins and Nutrition USMLE Step 1 Practice Question

An 83-year-old male is brought to the physician because of diarrhea and vomiting for the past month. He is unable to give a clear history, but his daughter reports that he has been 'quite sad' lately and often seems very confused. His diet consists of dried cereal and maize; he never eats milk or eggs. Physical examination shows sharply demarcated plaques on his hands, feet, and around his neck. Which of the following amino acids can substitute for a portion of the vitamin most likely to be deficient in this patient?