PAH and phenylketonuria: When phenylalanine hydroxylase defects occur, Phe generates phenylpyruvic acid, phenylacetic acid, and N-acetyl-L-phenylalanine metabolisms, and an abnormal accumulation of the byproducts of phenylalanine metabolism can be seen in the urine and plasma of phenylketonuria patients, which is the most common amino acid metabolism defect [26,27,28].