Which compound is elevated in Phenylketonuria?

Phenylketonuria (PKU) is a metabolic disorder caused by a deficiency in the enzyme phenylalanine hydroxylase. This enzyme is responsible for converting phenylalanine, an amino acid obtained from protein in the diet, into tyrosine. When this enzyme is defective or absent, the body cannot adequately convert phenylalanine, leading to an accumulation of this amino acid in the blood.

In individuals with PKU, the elevated levels of phenylalanine can cause various neurological problems if not managed properly, as high levels are toxic to the brain. Importantly, this elevated phenylalanine is what clinicians monitor when diagnosing and managing PKU.

While tyrosine is also an amino acid produced from phenylalanine, in PKU, its levels may actually be low or normal due to the impaired conversion process. Methionine and cysteine are unrelated amino acids that do not see a significant increase in PKU. Therefore, the correct answer highlights the accumulation of phenylalanine, which is the hallmark of this disorder.