PAH and phenylketonuria: In classic PKU, a defect in the phenylalanine 4-hydroxylase activity leads to a metabolic block in the conversion of phenylalanine to tyrosine, which results in abnormal metabolic product accumulation in the patient’s body in large quantities—phenylpyruvate, phenyllactate, and phenylacetate, the content of which is insignificant under normal conditions.