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
The most effective drug against extracellular mycobacteria is : 
Select and explain. 
A. Isoniazid, B. Rifampicin, C. Pyrazinamide, D. Ethambutol
B: Rifampicin
Answer: B: Rifampicin 
Explanation: 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 µg/ml whereas rifampicin inhibits the growth of bacteria at a concentration of 0.05 µg/ml.
Pyrazinamide acts more in acidic pH and it requires a concentration of 12.5 µg/ml.
Thus, a most active drug for extra-cellular bacteria is rifampicin.