This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
The primary deficiency in the disease phenylketonuria occurs in the synthesis of: 
Select and explain. 
A. Phenylpyruvate, B. Dihydroxyphenylalanine (DOPA), C. Phenylalanine, D. Tyrosine
D: Tyrosine
Answer: D: Tyrosine 
Explanation: 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.