PKU is a congenital amino acid metabolic disorder. In the following rare variants of PKU, dihydrobiopterin synthesis is affected. The enzyme deficient is

In Type 2 and 3 of PKU, there is deficiency of enzyme dihydrobiopterin reductase. Hence there is deficiency of dihydrobiopterin.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 236