This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
[{'content': 'The most effective drug against extracellular mycobacteria is : \nSelect and explain. \nA. Isoniazid, B. Rifampicin, C. Pyrazinamide, D. Ethambutol', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ethambutol is a bacteriostatic drug.\nINH and rifampicin are equally effective against intra as well as extracellular mycobacteria. INH require a concentration of 0.25\xa0µg/ml whereas rifampicin inhibits the growth of bacteria at a concentration of 0.05\xa0µg/ml.\nPyrazinamide acts more in acidic pH and it requires a concentration of 12.5\xa0µg/ml.\nThus, a most active drug for extra-cellular bacteria is rifampicin.', 'role': 'assistant'}]