A physician is examining a pediatric patient brought in by his parents because they are disturbed by his progressive mood changes and apparent heightened anxiety. In addition, the parents report that the child appears to have a high degree of sensitivity to sunlight, with a rash forming with only a few minutes of exposure. Physical examination shows signs of cerebellar ataxia that include unsteady gait and uncoordinated eye movements. Additional findings include a reddish rash on the child’s face, arms, and hands. Serum and urinalysis show significant aminoacidemia and aciduria. Which of the following would be the most useful dietary supplement in the treatment of this patient?