A newborn screening reveals elevated phenylalanine levels due to a deficiency of which enzyme?

- The newborn screening reveals elevated phenylalanine levels, which is characteristic of Phenylketonuria (PKU).
- PKU is caused by a deficiency of the enzyme phenylalanine hydroxylase, which is responsible for converting phenylalanine to tyrosine.
- When this enzyme is deficient, phenylalanine accumulates in the blood and tissues, leading to toxic effects on the brain if left untreated.

- In contrast, Maple syrup urine disease involves a defect in the metabolism of branched-chain amino acids (leucine, isoleucine, and valine), not phenylalanine.
- Tyrosinemia is related to defects in the breakdown of tyrosine, but it does not cause elevated phenylalanine levels.
- Homocystinuria involves a defect in the metabolism of methionine and homocysteine and is unrelated to phenylalanine metabolism.

Therefore, elevated phenylalanine levels detected during newborn screening specifically indicate a deficiency of phenylalanine hydroxylase, confirming the diagnosis of Phenylketonuria (PKU).

Reference: Harper's Illustrated Biochemistry, 31st Edition, Amino Acid Metabolism, Chapter 25