What is the most serious complication of untreated phenylketonuria (PKU) in infants?

The most serious complication of untreated phenylketonuria (PKU) in infants is intellectual disability. PKU is a genetic disorder characterized by the inability of the body to break down an amino acid called phenylalanine due to a deficiency in the enzyme phenylalanine hydroxylase. When untreated, phenylalanine accumulates to toxic levels in the body, particularly affecting the brain, which can lead to severe developmental delays and intellectual disabilities.

Early detection through newborn screening and management of PKU through a low-phenylalanine diet can prevent these severe outcomes. If dietary restrictions are adhered to, intellectual disability can often be avoided altogether, highlighting the critical importance of early intervention in managing this condition. While other complications, such as microcephaly and seizures, can occur in untreated PKU, they are generally not as severe or as impactful on overall functioning and quality of life as the risk of intellectual disability.