Which of the following is NOT a second-line injectable agent used for MDR-TB?

• A. Ethambutol
• B. Streptomycin
• C. Kanamycin
• D. Capreomycin

Answer: (A)

Second-line injectable agents for MDR-TB are drugs given by injection that are reserved for strains resistant to first-line therapy. The commonly used injectables in this group include kanamycin and capreomycin, which are specifically chosen for MDR-TB regimens. Streptomycin also appears as an injectable option in some MDR-TB contexts, though its role is more limited due to resistance and toxicity concerns.

Ethambutol, on the other hand, is a first-line drug taken orally and is not used as a second-line injectable. It targets the mycobacterial cell wall by inhibiting arabinosyl transferase, helping to prevent resistance when used in standard regimens, but it is not part of the injectable drugs selected for MDR-TB treatment. Therefore, it is the option that does not belong in the second-line injectable group.