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Phenylketonuria (PKU) is an inborn error of metabolism that can be diagnosed during the first days of life with routine newborn screening. PKU is characterized by absence or deficiency of an enzyme called phenylalanine hydroxylase (PAH), responsible for processing the amino acid phenylalanine. Amino acids are the chemical building blocks of proteins and are essential for proper growth and development. With normal PAH activity, phenylalanine is converted to another amino acid, tyrosine. However, when PAH is absent or deficient, phenylalanine accumulates and is toxic to the brain. Without treatment, most people with PKU would develop severe intellectual disability. To prevent intellectual disability, treatment consists of a carefully controlled, phenylalanine-restricted diet beginning during the first days or weeks of life.
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