Patients (5-18y) receiving standard clinical care at the BC Children's Hospital's Biochemical Diseases Clinic will be enrolled.The study will be conducted twice on each patient. Baseline measurements will be performed at the beginning of each study for each patient.The Physician in charge will then prescribe the standard dose of sapropterin dihydrochloride (Kuvan®) (20mg/kg/d) or otherwise as deemed appropriate by the physician-in-charge. The test will be repeated a week (minimum) after the beginning of treatment with the prescribed dose.