Phenylketonuria is caused by a mutation in the phenylalanine hydroxylase gene, resulting in reduced or lost enzyme activity, and a metabolic disorder of phenylalanine in the liver. Children with phenylketonuria lack phenylalanine hydroxylase, leading to reduced tyrosine and normal metabolic products. The content of phenylalanine in the blood increases, secondary metabolic pathways are enhanced, producing phenylpyruvic acid, phenylacetic acid, and phenyllactic acid, which are excreted in large amounts in the urine. High concentrations of phenylalanine and its abnormal metabolic products inhibit tyrosinase, leading to disturbances in melanin synthesis. Therefore, untreated children will gradually show changes in hair color from black to yellow and skin lightening after three months.