Phenylketonuria (PKU) is a metabolic disorder in which an inability to properly metabolize the amino acid phenylalanine (Phe) leads to a buildup of Phe in the blood and subsequent neurotoxicity that can cause intellectual disability, delayed speech, seizures, behavior abnormalities, and other medical and mental health problems if untreated. PKU is typically diagnosed soon after birth using biochemical tests that are performed after an abnormal newborn screening result. The most severe form of PKU, classic PKU, is typically characterized by blood Phe levels exceeding 1200 mol/L while on a...