PAH and phenylketonuria: A classic PKU caused by several mutations in the phenylalanine hydroxylase gene, the general result of which is a deficiency of the enzyme activity [17,19,20], and PKU forms, previously called atypical, which are associated with impaired metabolism of tetrahydrobiopterin (BH4), a coenzyme involved in the hydroxylation of several amino acids, including phenylalanine [21,22].