Most effective drug against extracellular mycobacteria -

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 ng/ml or less ii) Inhibitory concentration of INH -- 0.2 pg/m1 or less iii) Inhibitory concentration of streptomycin -- 1-10 ug/ml 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 ug/ml or less) ii) INH (inhibitory concentration 0.2 ug/ml or less) iii) Pyrazinamide (inhibitory concentration --) 20 pg/m1) 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.