<prompt> Allopurinol prevents conversion of : a Hypoxanthine to xanthine b Xanthine to hypoxanthine c Hypoxanthine to I.M.P. d Xanthine to uric acidabcdadbd  <response>: Xanthine oxidase catalyses the conversion of hypoxanthine to xanthine as well as xanthine to uric acid. This enzyme is inhibited by allopurinol. 