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
PKU is a congenital amino acid metabolic disorder. In one of the following rare variants of PKU Dihydro Biopterin synthesis is affected. The enzyme deficient is 
Select and explain. 
A. Histidine decarboxylase, B. Phenylalanine hydroxylase, C. Dihydropterin reductase, D. Tyrosine deficiency
C: Dihydropterin reductase
Answer: C: Dihydropterin reductase 
Explanation: Atypical form of phenylketonuria is due to deficiency of dihydrobiopterin reductase which is required for converion of H2-biopterin intorin. H4-biopterin is required for oxidation of phenylalanine because it is a cofactor for phenylalanine hydroxylase.