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Phenylketonuria (PKU)

Phenylketonuria (PKU) is an inherited disorder in which the body cannot break down an amino acid called phenylalanine, which is a part of protein. If treatment is not started soon after birth, phenylalanine levels rise and can cause lasting brain and nervous system damage, such as intellectual disability.

Symptoms of PKU usually appear within a few months of birth, after a baby has started drinking formula or breast milk and phenylalanine has built up in his or her blood. Before birth, the mother's body filters out the excess phenylalanine for the baby (fetus).

Early symptoms may include:

  • A musty odor to the skin, hair, and urine.
  • Weight loss from vomiting and frequent diarrhea.
  • Irritability.
  • Skin problems.
  • Sensitivity to light.

Screening for PKU is routinely done shortly after birth, making early diagnosis and treatment possible. Starting treatment within the first few weeks of birth may prevent lasting brain damage. In rare cases, some children who receive treatment will have learning or behavior problems. Left untreated, PKU causes progressively more severe intellectual disability.

People with PKU must follow a diet low in protein throughout life. Women of childbearing age with PKU must carefully manage their phenylalanine levels to prevent harm to their baby should they become pregnant. Babies born to mothers who have high phenylalanine levels during pregnancy are at risk for intellectual disability and other developmental problems.

Last Revised: September 8, 2011

Author: Healthwise Staff

Medical Review: John Pope, MD - Pediatrics & Chuck Norlin, MD, MD - Pediatrics

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