If the minimum inhibitory concentration (MIC) for sulfamethoxazole alone is 100 µg/mL and for trimethoprim alone is 50 µg/mL, and combined therapy allows a 10-fold reduction in the required amount of each drug to achieve the same antibacterial effect, what concentrations of each drug would be expected when used together?

• A. Sulfamethoxazole 20 µg/mL and trimethoprim 10 µg/mL, calculated by splitting the 10-fold reduction across both drugs equally but maintaining half of each single-agent MIC.
• B. Sulfamethoxazole 50 µg/mL and trimethoprim 25 µg/mL, reflecting a twofold reduction in required concentrations rather than tenfold.
• C. Sulfamethoxazole 1000 µg/mL and trimethoprim 500 µg/mL, because combined therapy requires more drug due to competition between mechanisms decreasing potency.
• D. Sulfamethoxazole 10 µg/mL and trimethoprim 5 µg/mL.

Answer: (D)