Hartnup disease

A 12-year-old girl comes to the emergency department because of a sudden onset rash across her body. The rash appeared after she played outside under the sun for 35 minutes. She also says she has double vision. Physical examination shows dry, scaly, well-marginated eruptions on the face, backs of the hands, and lower back. When asked to walk, her gait is wide and unsteady. Supplementation of which of the following nutrients is the most appropriate treatment?