Bactrim combines sulfamethoxazole and trimethoprim to inhibit what?

• A. It inhibits cell wall synthesis
• B. It inhibits DNA replication
• C. It inhibits sequential steps in folic acid synthesis
• D. It inhibits protein synthesis

Answer: (C)

Blocking two consecutive steps in bacterial folate synthesis is what makes Bactrim effective. Sulfamethoxazole inhibits dihydropteroate synthase, the enzyme that starts folate production by using PABA. Trimethoprim then inhibits dihydrofolate reductase, stopping the next step to make tetrahydrofolate. Without tetrahydrofolate, the bacteria can’t synthesize thymidine and other nucleotides needed for DNA replication, leading to bacterial death. Humans don’t synthesize folate the same way and obtain it from the diet, so this combo selectively targets bacteria. This is different from inhibiting cell wall synthesis, directly blocking DNA replication, or stopping protein synthesis, which are the actions of other antibiotic classes.