Ans. B Serotonin
Explanation: BH4 is a cofactor used by hydroxylase enzymes in the synthesis of tyrosine, dopa, and serotonin, as well as nitric oxide. Serotonin is synthesized from tryptophan, and the initial step in this reaction is catalyzed by an enzyme that uses BH4 as a cofactor. Enzyme dihydrobiopterin reductase deficiency causes defective regeneration of BH4, and is an uncommon cause of phenylketonuria (PKU).