Phenylketonuria (PKU), is a genetic disorder that results in the deficiency of an enzyme that converts phenylalanine, an essential amino acid, to tyrosine. An excess of phenylalanine in the blood can cause severe mental retardation. It is tested for in newborn infants; affected children who are detected and treated are less likely to develop neurological problems or have seizures and mental retardation, though such clinical disorders are still possible.