Phenylketonuria is an autosomal recessive genetic disorder. Phenylalanine, which is an essential amino acid required by the human body, is partly used for protein synthesis after ingestion and partly converted into tyrosine via the action of phenylalanine hydroxylase, which is necessary for the synthesis of substances such as adrenaline, melanin, and thyroxine. Phenylketonuria is mainly caused by a deficiency of phenylalanine hydroxylase, which prevents the conversion of phenylalanine to tyrosine. This leads to increased levels of phenylalanine in the blood, cerebrospinal fluid, and various tissues. Additionally, as the primary metabolic pathway is blocked, secondary metabolic pathways are enhanced. Under the action of transaminases, phenylalanine undergoes deamination to produce large quantities of phenylpyruvic acid, which is further metabolized to produce phenylacetic acid, phenyllactic acid, and p-hydroxyphenylpyruvic acid, among other byproduct metabolites. These are excreted in large amounts in the urine. The high concentration of phenylalanine and its byproduct metabolites accumulates in brain tissue, leading to damage to brain cells and resulting in a range of clinical symptoms.