9-Methyl-1,2,3,9-tetrahydro-4H-carbazol-4-one ; 2,3-Dihydro-9-methylcarbazol-4(1H)-one; 1,2,3,9-Tetrahydro-9-methyl-4H -carbazol-4-one; Ondansetron Related Compound C (USP); Ondansetron EP Impurity C ;