Phenylketonuria is caused by a decrease in melanin synthesis due to inhibition of tyrosinase, resulting in a light and brown hair color in children. Phenylketonuria (PKU) is a genetic disorder in which phenylalanine metabolism is impaired due to a deficiency or reduced activity of phenylalanine hydroxylase (PAH) in the liver. It is more common in inherited amino acid metabolism deficiency disorders. The amount of phenylalanine ingested for protein synthesis decreases with age. After birth, the daily intake of phenylalanine is about 0.5 g, increasing to 4 g in children and adults. a larger portion of this is oxidized to tyrosine, a process that depends mainly on phenylalanine hydroxylase (PAH), but also requires the involvement of cofactors. If this oxidation process is impaired, there is an accumulation of phenylalanine in the body, in which case it is metabolized by other pathways to produce the harmful substance phenylketonate. Phenylketonuria (PKU) is a genetic disorder caused by reduced or absent PAH activity, which can also lead to decreased melanogenesis due to tyrosine inhibition and accumulation of hydroxyphenylpyruvate due to hydroxyphenylpyruvase inhibition. The disease is autosomal recessive, with the mutated gene located on the long arm of chromosome 12 (12q24.1), and a small variation in this gene can cause the disease, not due to a gene deletion. It is a hereditary disease caused by the marriage of two heterozygotes and is most common in the offspring of consanguineous marriages, with approximately 40% of the affected siblings affected. The basic biochemical abnormality of the disease is a deficiency of phenylalanine hydroxylase in the liver due to a mutation in the phenylalanine hydroxylase gene. The severity of the clinical manifestations varies greatly if the base pairs mutated are different, and may present as typical PKU or mild hyperphenylalaninemia.