Phenylketonuria (PKU) is a rare genetic disorder that affects the way the body processes the amino acid phenylalanine. People with PKU are unable to properly break down phenylalanine, which can lead to a buildup of toxins in the body that can cause brain damage and other serious health problems. PKU is typically diagnosed shortly after birth through a simple blood test and can be managed with a strict low-phenylalanine diet.