The first anti-infective drug mentioned is categorized as which class?

• A. Sulfa
• B. Penicillin
• C. Erythromycin
• D. Tetracycline

Answer: (A)

Sulfonamides are a distinct class of anti-infectives that work by blocking bacterial folate synthesis. They inhibit dihydropteroate synthase, preventing the formation of dihydrofolic acid from PABA, which bacteria need to make DNA and RNA. Humans don’t make folate and obtain it from the diet, so this target is selective for bacteria, often resulting in a bacteriostatic effect. When combined with trimethoprim, they can exert a bactericidal effect by blocking a second step in the same pathway. That mechanism is why the first drug mentioned falls into the sulfonamide class. By comparison, penicillins inhibit cell wall synthesis, erythromycin (a macrolide) blocks the 50S ribosomal subunit, and tetracyclines inhibit the 30S ribosomal subunit.