The primary deficiency in the disease phenylketonuria occurs in the synthesis of:

Phenylalanine hydroxylase which conves phenylalanine to tyrosine is deficient in phenylketonuria. The most common cause of hyperphenylalaninemia is deficiency of the enzyme phenylalanine hydroxylase, which catalyzes the conversion of phenylalanine to tyrosine. Ref: Barsh G. (2010). Chapter 2. Genetic Disease. In S.J. McPhee, G.D. Hammer (Eds), Pathophysiology of Disease, 6e.