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 phenylalanine has built up in a baby's 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.

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.