any of several autosomal recessive defects in the hydroxylation of phenylalanine resulting in accumulation and excretion of dietary phenylalanine; most commonly the defect is in the enzyme phenylalanine 4-monooxygenase; the most severe manifestation of this is classic PHENYLKETONURIA, but two benign forms also occur; rarely the defect is one of tetrahydrobiopterin metabolism.
