Which disorder is characterized by the body's inability to break down the amino acid phenylalanine?

Phenylketonuria (PKU) is characterized by the body's inability to properly metabolize the amino acid phenylalanine due to a deficiency in the enzyme phenylalanine hydroxylase. This metabolic disorder leads to a toxic buildup of phenylalanine in the body, which can result in severe intellectual disability and other neurological issues if left untreated. Individuals with PKU must adhere to a strict diet that limits phenylalanine intake to prevent these adverse effects.

In contrast, Fragile X Syndrome, Down Syndrome, and Autism Spectrum Disorder are genetic or neurodevelopmental conditions that involve different biological mechanisms and symptoms. Fragile X Syndrome is linked to a mutation on the X chromosome affecting intellectual development. Down Syndrome is a genetic disorder caused by an extra chromosome 21, which leads to characteristic physical and developmental features. Autism Spectrum Disorder encompasses a range of conditions with social, communication, and behavioral challenges, but it is not related to the metabolism of phenylalanine. Thus, PKU is uniquely defined by its specific metabolic dysfunction involving phenylalanine.