## Why option 1 is right Trimethoprim-sulfamethoxazole (TMP-SMX, cotrimoxazole) is the drug of choice for both treatment and prophylaxis of Pneumocystis jirovecii pneumonia. The mechanism is synergistic blockade of folate synthesis: trimethoprim inhibits dihydrofolate reductase (DHFR), while sulfamethoxazole inhibits dihydropteroate synthase (DHPS). This sequential inhibition of two steps in the folate pathway is particularly effective against P. jirovecii, which cannot salvage preformed folate and must synthesize it de novo. The treatment dose of 15–20 mg/kg/day in 3–4 divided doses for 21 days is standard for acute PCP (Murray 9e; Harrison 21e Ch 218). ## Why each distractor is wrong - **Option 2 (bacterial cell wall synthesis)**: P. jirovecii is a fungus, not a bacterium, and does not have peptidoglycan cell walls. This describes the mechanism of beta-lactams and vancomycin, not TMP-SMX. - **Option 3 (ergosterol binding)**: While ergosterol is a component of fungal cell membranes, TMP-SMX does not act via direct membrane disruption. This mechanism describes polyene antifungals such as amphotericin B. - **Option 4 (DNA gyrase inhibition)**: This is the mechanism of fluoroquinolones, not TMP-SMX. P. jirovecii is susceptible to TMP-SMX because of its dependence on de novo folate synthesis, not DNA gyrase inhibition. **High-Yield:** TMP-SMX is the only drug that is both first-line treatment AND standard prophylaxis for PCP; its synergistic two-step folate blockade is uniquely effective against P. jirovecii's inability to salvage preformed folate. [cite: Murray 9e; Harrison 21e Ch 218]
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