Phenylketonuria (fen-ul-ke-toe-NU-re-uh) is a birth defect that causes an amino acid called phenylalanine to build up in your body. Phenylketonuria is caused by a mutation in a gene that helps create the enzyme needed to break down phenylalanine.

Amino acids are the building blocks for protein, but too much phenylalanine can cause a variety of health problems. People with phenylketonuria (PKU) — babies, children and adults — need to follow a diet that limits phenylalanine, which is found mostly in foods that contain protein.

Babies in the United States and many other countries are screened for phenylketonuria soon after birth. Although phenylketonuria is rare, recognizing phenylketonuria right away can help prevent serious health problems.

Nov. 17, 2011

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