Phenylketonuria (PKU) is an inherited metabolic disorder caused by a recessive gene, meaning a child must inherit a copy of the mutated allele from each parent to develop the condition. Individuals with PKU lack the enzyme needed to metabolize the amino acid phenylalanine. If the condition is not treated through a regulated diet, the resulting buildup of phenylalanine in the brain becomes toxic, leading to severe consequences such as significant deficits in cognitive function, intellectual disability, seizures, and an increased risk of various psychiatric disorders.