Phenylketonuria (PKU) is a rare, inherited metabolic disease in which a baby is born without the ability to properly break down an amino acid called phenylalanine. When levels of phenylalanine build up to a toxic level in your child’s body, damage can occur in the central nervous system and the brain. Infants with the condition may experience delayed mental and social skills, hyperactivity, seizures and skin rashes. They also tend to have lighter skin, hair and eyes, as phenylalanine also plays a role in the body’s production of melanin. The buildup of phenylalanine may also cause your child’s breath, skin and urine to have a musty odor. Fortunately, babies with these complications are rare because babies are usually screened for PKU with a heel-prick test soon after birth and treatment is immediately started. With treatment, babies can live nearly normal lives.