The cause of phenylketonuria is that phenylalanine is an essential amino acid for the human body. Some of the phenylalanine ingested is used for protein synthesis, while another part is converted into tyrosine by the action of phenylalanine hydroxylase. Only a small amount of phenylalanine undergoes a secondary metabolic pathway and is converted into phenylpyruvate under the action of transaminase. Due to the reduced activity of phenylalanine hydroxylase in affected children, phenylalanine cannot be converted into tyrosine, leading to an extremely high concentration of phenylalanine in blood, cerebrospinal fluid, and tissues. This, through a bypass metabolic pathway, results in the production of a large amount of phenylpyruvate, phenylacetate, phenyllactate, and p-hydroxyphenylacetate. The high concentration of phenylalanine and its metabolic products then leads to brain damage, which is the cause of the disease.