PAH and pulmonary arterial hypertension: Phenylketonuria (PKU) is a rare autosomal recessive inborn error of phenylalanine (Phe) metabolism, caused by a mutation in the phenylalanine hydroxylase gene (PAH, cDNA sequence, GenBank U49897) encoding the enzyme L-phenylalanine hydroxylase (EC 1.14.16.1) [1].