Young children with phenylketonuria are mainly characterized by intellectual backwardness, light-colored hair and skin, and urine with a ratty odor. Phenylketonuria, is caused by a mutation in the phenylalanine hydroxylase gene, which reduces or lacks enzyme activity. Phenylalanine is an essential amino acid, which is converted to tyrosine under the action of phenylalanine hydroxylase, but young children with phenylketonuria cannot convert phenylalanine to tyrosine, resulting in an increased concentration of phenylalanine in the body. 1. intellectual backwardness: it is a prominent manifestation of this disease. Increased concentration of phenylalanine in the cerebrospinal fluid affects the development of the central nervous system of young children, which can cause intellectual backwardness. 2. Light color of hair and skin: high concentration of phenylalanine in the body can inhibit tyrosinase and reduce melanin synthesis, showing light color of hair, yellow or brownish-yellow, and fairer skin. 3. Urine with rat odor: Phenylalanine will stimulate the development of transaminase, so that the urine is discharged in large quantities of phenylpyruvic acid, phenylacetic acid and phenyl lactic acid, with a special rat urine odor. If you find that your child has any of the above discomfort symptoms, it is recommended to consult a doctor in time.