What does the MBC test specifically assess about a drug's activity?

• A. The time to kill
• B. The killing effect on a specific patient isolate
• C. The bactericidal effect
• D. The mutation rate

Answer: (B)

The main idea is that the MBC measures whether a drug actually kills the bacteria, and it does this for a specific bacterial isolate. In practice, after determining the MIC, you test whether the bacteria can be recovered after exposure to the antibiotic at that concentration or higher. This is done by subculturing from the growth-inhibiting tube(s) onto drug-free medium and seeing if colonies form. If colonies fail to grow, the concentration is bactericidal for that isolate; if colonies do grow, it was only inhibitory, not killing. The MBC is defined as the lowest concentration that achieves a 99.9% (3-log) reduction in viable organisms compared with the starting inoculum.

This is why the correct choice emphasizes the killing effect on a specific patient isolate. It distinguishes killing (bactericidal activity) from merely inhibiting growth and from unrelated concepts like mutation rate or time-to-kill kinetics.