A 2-year-old boy is brought to clinic because of poor development as well as vomiting, irritability, and a skin rash. The boy's mother also notes that his urine has a strange "mousy" odor. Physical examination reveals the child has an eczema-like rash, is hyperreflexive, and has increased muscle tone. He has a surprisingly fair-skinned complexion compared to the rest of his family. What is the most likely diagnosis?