A genetic disorder in which there is disordered metabolism of the amino acid phenylalanine, leading to severe mental retardation of affected children. The disease is caused by the absence or deficiency of the enzyme phenylalanine hydroxylase, which results in the accumulation of phenylalanine in all body fluids. There are also high levels of the ketone phenylpyruvate in the urine, hence the name of the disease. The disease occurs in individuals who are homozygous for the defective recessive allele at the PAH gene on chromosome 12. Both parents of such individuals are thus heterozygous carriers of the allele. The advent of gene probes has greatly aided accurate diagnosis, of both phenylketonurics and carriers.