Houston Methodist Pharmacology Practice Exam

Trimethoprim-sulfamethoxazole is used to treat bacterial infections by inhibiting folic acid synthesis, which is essential for bacterial growth and replication. This medication combines two different mechanisms of action: sulfamethoxazole is a sulfonamide that inhibits the enzyme dihydropteroate synthase, interfering with the synthesis of dihydrofolate, while trimethoprim inhibits dihydrofolate reductase, blocking the conversion of dihydrofolate to tetrahydrofolate. By targeting two distinct steps in the folate synthesis pathway, this combination enhances the antimicrobial activity and can be more effective than using either agent alone.

The other medications listed have different mechanisms of action. Ciprofloxacin, for example, is a fluoroquinolone that inhibits bacterial DNA gyrase and topoisomerase IV, which are critical for bacterial DNA replication. Amoxicillin is a penicillin antibiotic that inhibits bacterial cell wall synthesis by targeting penicillin-binding proteins. Azithromycin, a macrolide, works by inhibiting bacterial protein synthesis by binding to the 50S ribosomal subunit. Each of these alternative medications addresses bacterial infection through mechanisms unrelated to folic acid synthesis.