Phenylketonuria (PKU)'s an autosomal recessive genetic disorder characterized by a deficiency in the enzyme hepatic phenylalanine hydroxylase. This enzyme's necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. When PAH's deficient, phenylalanine accumulates and's converted into phenylpyruvate (also known as phenylketone), which's detected in the urine. Left untreated, this condition can cause problems with brain development, leading to progressive mental retardation,… (More on Phenylketonuria)