<prompt> Hyperphenylalaninemia is due to defect in the enzyme:Phenylalanine hydroxylase:TyrosinaseHomogentisic acid oxidaseOrnithine transcarbamylase  <response>: a. Phenylalanine hydroxylaseRef: Nelsons 20e p 636-636Deficiency of the enzyme phenylalanine hydroxylase or of its cofactor tetrahydrobiopterin BH4 causes accumulation of phenylalanine in body fluids and in the brain. 