Which combination is formed by Trimethoprim and Sulfamethoxazole?

• A. Co-Trimazine
• B. Co-Amoxiclav
• C. Co-Trimethoprim
• D. Co-Trimoxazole

Answer: (D)

The two drugs form a fixed-dose combination known as co-trimoxazole. This pairing is named to reflect that trimethoprim and sulfamethoxazole are combined into one product and work together as a unit. Mechanistically, sulfamethoxazole inhibits dihydropteroate synthase, blocking the first step of bacterial folate synthesis, while trimethoprim blocks dihydrofolate reductase, inhibiting the next step. By blocking sequential steps in the same pathway, they produce a strong synergistic bactericidal effect, which is why this combination is widely used. In this context, co-trimoxazole (also called trimethoprim-sulfamethoxazole) is the standard way to refer to the two drugs together, matching the given answer.