The main cause of neonatal phenylalanine hypertension is phenylalaninuria, an autosomal recessive disorder. In newborns, phenylalanine hydroxylase activity in the liver is significantly reduced, leading to a deficiency of the enzyme in the liver cells and a failure to convert phenylalanine to tyrosine, resulting in a very high concentration of phenylalanine in the blood, cerebrospinal fluid, and urine, leading to hyperphenylalaninuria and clinical symptoms. Initially, vomiting and irritability occur, and neuromotor development is severely delayed. In untreated cases, there is often a speech impairment, especially in the first year of life, and sometimes seizures.