PAH (phenylalanine hydroxylase) is a cytosolic enzyme that catalyzes the hydroxylation of L-phenylalanine to L-tyrosine, serving as a critical step in phenylalanine catabolism and tyrosine biosynthesis. This conversion is essential for amino acid metabolism and supports downstream biosynthetic pathways including catecholamine production. PAH deficiency causes phenylalanine hydroxylase deficiency, a genetic disorder characterized by impaired conversion of phenylalanine to tyrosine, leading to elevated plasma phenylalanine levels and potential neurodevelopmental consequences if untreated. Clinically, phenylalanine hydroxylase deficiency is managed through early detection via newborn screening and dietary restriction of phenylalanine intake, which prevents accumulation of this amino acid and its toxic metabolites. The discovery and implementation of screening programs for this metabolic disorder represents a major success in preventive medicine, as early intervention can eliminate or substantially reduce the neurological complications historically associated with untreated disease. PAH variants have also been identified in genetic association studies, though the functional consequences of specific variants in other disease contexts remain to be fully elucidated.