In the treatment of drug-susceptible TB, which four drugs constitute the initial regimen?

• A. Isoniazid, rifampin, pyrazinamide, ethambutol.
• B. Isoniazid, rifampin, ethambutol, streptomycin.
• C. Pyrazinamide, ethambutol, rifapentine, rifampin.
• D. Isoniazid, ethambutol, rifabutin, pyrazinamide.

Answer: (A)

Treating drug-susceptible tuberculosis starts with using four first-line drugs together to prevent resistance and achieve rapid, effective kill of the bacilli. This intensive four-drug regimen—rifampin, isoniazid, pyrazinamide, and ethambutol—is given for about two months. Each drug adds a different effect: rifampin and isoniazid provide strong bactericidal activity, pyrazinamide is particularly useful for shortening therapy by targeting semi-dormant organisms in acidic environments, and ethambutol helps prevent the emergence of resistance while susceptibility results are pending. After the initial two months, if the organism is susceptible to these drugs, pyrazinamide and ethambutol can be stopped, and therapy continues with rifampin and isoniazid for a total of about six months.

Choice alternatives replace one of these drugs with streptomycin, rifabutin, or rifapentine. Streptomycin is not part of the standard initial regimen for drug-susceptible TB, rifabutin is used in specific scenarios (often with HIV or different drug interactions), and rifapentine is not a standard component of the initial four-drug intensive phase for drug-susceptible TB.