<prompt> The most effective drug against extracellular mycobacteria is :IsoniazidRifampicinPyrazinamideEthambutol  <response>: Ethambutol is a bacteriostatic drug.INH and rifampicin are equally effective against intra as well as extracellular mycobacteria. INH require a concentration of 0.25 μgml whereas rifampicin inhibits the growth of bacteria at a concentration of 0.05 μgml.Pyrazinamide acts more in acidic pH and it requires a concentration of 12.5 μgml.Thus, a most active drug for extra-cellular bacteria is rifampicin. 