Phenylketonuria is a hereditary metabolic disease caused by a deficiency or low activity of phenylalanine hydroxylase. Its main clinical manifestations include very obvious developmental delays in children and significantly lower IQ than their peers, severely affecting the child's growth and development. Due to the lack of this enzyme, the increase of abnormal metabolic products affects brain function, leading to symptoms such as seizures, convulsions, increased muscle tone, as well as hyperactivity and restlessness in children. The skin becomes particularly dry, prone to eczema, appears pale, and the hair turns yellow. The urine has a distinct mouse-like odor. Once diagnosed, early treatment is essential to reduce the sequelae of brain damage. The earlier the treatment, the better the prognosis. For these children, a special formula is needed, which is low in phenylalanine.