Phenylketonuria
![](/Images/godic/202502/02/L-Phenylalanin_-_L-Phenylalanine.svg3140.png")
![Blood is taken from a two-week-old infant to test for phenylketonuria](/Images/godic/202502/02/Phenylketonuria_testing3140.jpg")
![Phenylketonuria is inherited in an autosomal recessive fashion](/Images/godic/202502/02/Autorecessive.svg3140.png")
![Pathophysiology of phenylketonuria, which is due to the absence of functional phenylalanine hydroxylase (classical subtype) or functional enzymes for the recycling of tetrahydrobiopterin (new variant subtype) utilized in the first step of the metabolic pathway.](/Images/godic/202502/02/Pathophysiology_of_metabolic_disorders_of_phenylalanine_and_tyrosine3140.png")
Phenylketonuria (PKU) is an inborn error of metabolism involving impaired metabolism of the amino acid phenylalanine. Phenylketonuria is caused by absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity. The condition is also known as phenylalanine hydroxylase deficiency.