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Phenylketonuria

  • Definition

    Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine.


    Alternative Names

    PKU; Neonatal phenylketonuria


    Causes, incidence, and risk factors

    Phenylketonuria (PKU) is inherited, which means it is passed down through families. Both parents must pass on the defective gene in order for a baby to have the condition. This is called an autosomal recessive trait.

    Babies with PKU are missing an enzyme called phenylalanine hydroxylase, which is needed to break down an essential amino acid called phenylalanine. The substance is found in foods that contain protein.

    Without the enzyme, levels of phenylalanine and two closely-related substances build up in the body. These substances are harmful to the central nervous system and cause brain damage.