<s>[INST] context: R ifampin (Rifampicin, R): It is a semisynthetic derivative of rifamycin B obtained from Streptomyces mediterranei. Rifampin is bactericidal to M. tu berculosis and many other gram-positive and gram-negative bacteria like Staph. aureus, N. meningitidis, H. influenzae, E . coli, Klebsiella, Pseudomonas, Proteus and Legionella. Against TB bacilli, it is as efficacious as INH and better than all other drugs. The bactericidal action covers all subpopulations of TB bacilli, but acts best on slowly or intermittently (spuers) dividing ones, as well as on many atypical mycobacteri a . Both extra- and intracellular organisms are affected. It has good sterilizing and resistance preventing actions. Isoniazid (lsonicotinic acid hydrazide, H) Isoniazid is the antitubercular drug parexcellence, and an essential component of all antitubercular regimens, unless the patient is not able to tolerate it or bacilli are resistant. lt is primarily tuberculocidal. Fast multiplying organisms are rapidly killed, but quiescent ones are only inhibited. It acts on extracellular as well as on intracellular TB (bacilli present within macrophages); is equally active in acidic and alkaline medium. It is one of the cheapest antitubercular drugs. However, most atypical mycobacteria are not inhibited by INH. Ethambutol (E) Ethambutol is selectively tuberculostatic and clinically as active as S. Fast multiplying bacilli are more susceptible as are many atypical mycobacteria PYRAZINAMIDE:It is weakly tuberculocidal but more active in acidic medium. It is more lethal to intracellularly located bacilli and to those at sites showing an inflammatory response (pH is acidic at both these locations). ESSENTIALS OD MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:740,741,742 [/INST] question: Most effective drug against extracellular mycobacteria is: | options: A: Isoniazid B: Rifampicin C: Pyrazinamide D: Ethambutol | answer: B </s>