Which one of the following drugs is considered as primary antimycobacterial therapy?

• A. Isoniazid
• B. Kanamycin
• C. Amikacin
• D. Ciprofloxacin

Answer: (A)

Isoniazid is the mainstay of initial anti-TB therapy because it is highly potent against actively growing Mycobacterium tuberculosis and is specifically designed to disrupt the bacterium’s cell wall. It must be activated by the bacterial enzyme KatG, and once activated it blocks the synthesis of mycolic acids, essential components of the mycobacterial cell wall. This rapid, bactericidal action against actively dividing bacteria makes it the cornerstone of standard first-line regimens (together with rifampin, pyrazinamide, and ethambutol). The other drugs listed—kanamycin and amikacin, which are aminoglycosides used mainly for resistant TB, and ciprofloxacin, a fluoroquinolone used as a second-line option—do not serve as the initial primary therapy for drug-susceptible TB.