Phenylketonuria (PKU) is an inherited metabolic disorder that causes an amino acid called phenylalanine to build up in the blood. Phenylalanine is a fundamental building block of proteins, obtained through diet, and is present in all protein-containing foods and some artificial sweeteners. If left untreated, the accumulation of high levels of phenylalanine in the body can lead to severe intellectual disabilities and other neurological problems.