PKU is a congenital amino acid metabotic disorder. In one of the following rare variants of PKU Dihydro Biopterin synthesis is affected. The enzyme deficient is: (PGI June 2008)

Ans C (Dihydropterin reductase) PhenylketonuriaPhenylketonuria is most commonQ clinically encountered inborn error of amino acid metabolism and characterized by accumulation of phenylalanineQ (and deficiency' of tyrosine caused by deficiency of phenylalanine hydroxylaseQ.Dihydropteridine reductase sysnthesizes BH2 is also required for tyrosine and tryptophan hydroxylase which catalyzes the reaction leading to synthesis of neurotransmitters such as serotonin and catecholamine.Table (Harrison 17th/2471-72):Inherited Disorders of AminoAcid MetabolismAmino - acid(s)ConditionEnzyme DefectClinical FindingsInheritanceaPhenylalanineQPhenylketonuria type 1Phenyl - alanineHydroxyl -laseQMental retardation'Q, microcephalyQ, hypopig- mented skin and hairs, eczema, "mousy" odorARQ Phenylketonuria type IIDihy - drop - teridine reductaseQMental retardation, hypotonia, spasticity, myoclonusAR Phenylketonuria type Hi6-Py - ruvayl - tetrahy - dropterin synthaseQDystonia, neurologic deterioration, seizures, mental retardationARTypes of PhenylketonuriaPhenylalanine hydroxylaseQ - Mental retardation, Microcephaly, hypopigmentation, eczema, Mousy odourDihydropteridin reductaseQ - Mental retardation, hypotonia, spasticity. Myoclonus.6-pyruvoyi-tetrahydropterin Synthase- Dystonia, Neurologic deterioration, Seizure, Mental retardation.