Phenylketonuria

Phenylketonuria (PKU) is a rare inherited disorder that occurs when the body is unable to break down the amino acid phenylalanine:
Cause
PKU is caused by mutations in the phenylalanine hydroxylase (PAH) gene, which provides instructions for making the enzyme that converts phenylalanine into other substances.
Mechanism
When the PAH gene is defective, the body can't break down phenylalanine, which builds up in the blood and other tissues. This can lead to toxic levels of phenylalanine, which can damage the brain and nervous system.
Symptoms
Untreated PKU can cause learning disabilities, behavioral difficulties, and a mousy odor in the urine and sweat.
Treatment
Early treatment can prevent symptoms.
The severity of PKU depends on the level of PAH activity:
Classic PKU: The most severe form of PKU, which occurs when there is no or very low PAH activity.
Variant PKU: A milder form of PKU that occurs when the PAH enzyme retains some activity.
Non-PKU hyperphenylalaninemia: A milder form of PKU that occurs when there is a variant in the PAH gene.