What causes Phenylketonuria?

Phenylketonuria (PKU) is an inherited disorder from autosomal recessive gene for metabolism.
 There is mutation in the gene that produces phenylalanine hydroxylase (PAH). PAH is used to break down phenylalanine. Phenylalanine is an amino acid that can be found in many food with protein. The mutation can cause reduction in the enzyme activity or not functioning at all, depending on the severity. When PAH is not working properly, phenylalanine cannot be processed efficiently and will begin to accumulate to toxic level in the human blood and tissue.

Reference:
(2012). Phenylketonuria. Retrieved June 21, 2012, from Wikipedia, the free encyclopedia Web site: http://en.wikipedia.org/wiki/Phenylketonuria#Metabolic_pathways.

(2012). Phenylketonuria. Retrieved June 21, 2012, from Genetics Home Reference, your guide to understand genetic conditions: http://ghr.nlm.nih.gov/condition/phenylketonuria.