<prompt> Most effective drug against extracellular mycobacteria -IsoniazidRifampicinPyrazinamideEthambutol  <response>: Ans. is a i.e., Isoniazid Antitubercular activity of drugs o INH rifampin is active against extracellular as well as intracellular organisms. o Pyrazinamide is more active against intracellular organism. o Streptomycin acts only on extracellular bacilli. o So, first line antitubercular drug acting on extracellular mycobacteria: i Rifampicin ii EVH iii Streptomycin o Now, question arises that which amongst the three is most effective ? o For this we will have to see inhibitory concentration in vitro for mycobacterium tuberculosis i Inhibitory concentration of rifampicin --1 ngml or less ii Inhibitory concentration of INH -- 0.2 pgm1 or less iii Inhibitory concentration of streptomycin -- 1-10 ugml o As inhibitory concentration of INH is least -- INH is most effective drug against extracellular mycobacteria. o Drugs acting on intracellular mycobaceria : i Rifampicin Inhibitory concentration -- l ugml or less ii INH inhibitory concentration 0.2 ugml or less iii Pyrazinamide inhibitory concentration -- 20 pgm1 So, INH is the most effective drug against intracellular mycobacteria too. About option d o Ethambutol is tuberculostatic, whereas other four first line antitubercular drugs are tuberculocidal. So, ethembutol is least effective first line antitubercular drug. 