Which metabolic disorder is characterized by high levels of phenylalanine?

Phenylketonuria (PKU) is a metabolic disorder caused by a deficiency in the enzyme phenylalanine hydroxylase, which is necessary for the conversion of phenylalanine to tyrosine. When this enzyme is deficient or absent, phenylalanine accumulates in the body, leading to high levels of this amino acid in the blood. Over time, elevated phenylalanine levels can lead to severe intellectual disability and other neurological issues if not managed through dietary restrictions.

In contrast, the other mentioned conditions do not primarily involve elevated phenylalanine levels. Cystic fibrosis is characterized by a defect in the CFTR gene that affects mucus production and primarily impacts the lungs and digestive system. Galactosemia involves an inability to metabolize galactose due to enzyme deficiencies related to galactose metabolism. Maple syrup urine disease is characterized by elevated levels of branched-chain amino acids due to an enzyme deficiency distinct from that of PKU.

Thus, the defining feature of PKU is the accumulation of phenylalanine, confirming that the correct answer is indeed PKU.