Why does mental retardation occur in phenylketonuria?

Mental retardation occurs in phenylketonuria (PKU) because of a deficiency in the enzyme phenylalanine hydroxylase, which leads to the toxic accumulation of phenylalanine in the brain. Elevated phenylalanine levels, particularly during critical periods of brain development, disrupt normal neurological processes, including the balance of neurotransmitters like dopamine and serotonin. This disruption results in cognitive and intellectual impairments, including delayed speech and language development, learning difficulties, and behavioral problems.