Co-Trimoxazole is the combination of which drugs?

• A. Amoxicillin + Clavulanic acid
• B. Trimethoprim and Sulfamethoxazole
• C. Trimethoprim and Sulfadiazine
• D. Rifampicin and Isoniazid

Answer: (B)

Co-trimoxazole is the fixed combination of trimethoprim and sulfamethoxazole in a 1:5 ratio. The reason this pairing is used is that it blocks two steps in bacterial folate synthesis: sulfamethoxazole inhibits dihydropteroate synthase, preventing production of dihydropteroic acid, while trimethoprim inhibits dihydrofolate reductase, preventing conversion to tetrahydrofolate. This sequential blockade leads to a synergistic bactericidal effect because bacteria cannot synthesize thymidine for DNA replication, while human cells obtain folate from the diet and are less affected. Other drug pairings listed involve different classes or combinations not matching this fixed TMP-SMX formulation, so they are not co-trimoxazole.