<prompt> Dihydrobiopterin is used in management of which amino acid defect?AlanineTyrosinePhenylalanineTryptophan  <response>: Ans. c PhenylalanineRef: Lippincotts Biochemistry, pg. 268Phenylketonuria PKU, caused by a deficiency of phenylalanine hydroxylase and is the most common clinically encountered inborn error of amino acid metabolism prevalence 1:11,000. Hyperphenylalaninemia may also be caused by deficiencies in the enzymes that synthesize or reduce the coenzyme tetrahydrobiopterin BH4 . It is important to distinguish among the various forms of hyperphenylalaninemia, because their clinical management is different. For example, a small fraction of PKU is a result of a deficiency in either dihydropteridine BH2 reductase or BH2 synthetase. These mutations prevent synthesis of BH4 , and indirectly raise phenylalanine concentrations, because phenylalanine hydroxylase requires BH4 as a coenzyme. BH4 is also required for tyrosine hydroxylase and tryptophan hydroxylase, which catalyze reactions leading to the synthesis of neurotransmitters, such as serotonin and catecholamines. 