A patient receives Drug X and Drug Y. In vitro testing shows the combination kills 10-fold more bacteria than predicted from independent effects of each drug. Which term best describes this interaction and why?

• A. Synergism, because the observed combined killing exceeds the expected additive effect of the two drugs.
• B. Antagonism, because the combination produced a measurable effect that differs from the effect of each drug alone, indicating interference.
• C. Indifference, because the combined effect must be exactly additive to be called synergy and any multiplicative increase is considered statistically insignificant.
• D. Tolerance, because bacteria can rapidly adapt to two drugs and show increased rate of killing only transiently before resistance emerges, which is the hallmark of antagonism rather than synergy.

Answer: (A)