**Question:** Drug of choice for treatment of Pneumocystitis jiroveci pneumonia is

A. Trimethoprim-sulfamethoxazole (TMP-SMX)
B. Azithromycin
C. Fluconazole
D. Amphotericin B

**Correct Answer:**

**Core Concept:** Pneumocystis jiroveci pneumonia (PCP) is a severe opportunistic infection in immunocompromised patients, primarily caused by the protozoan Pneumocystis jiroveci. TMP-SMX is a combination of two drugs that target different aspects of the Pneumocystis life cycle, making it particularly effective against the infection.

**Why the Correct Answer is Right:**

A. Trimethoprim-sulfamethoxazole (TMP-SMX): TMP is a folic acid antagonist that inhibits dihydrofolate reductase, and SMX is a diuretic that inhibits dihydropteroate synthase. By inhibiting these enzymes, TMP-SMX interferes with the folic acid synthesis pathway, ultimately inhibiting the growth and proliferation of Pneumocystis jiroveci. This combination is highly effective against PCP due to its dual mechanism of action.

**Why Each Wrong Option is Incorrect:**

B. Azithromycin: Azithromycin is a macrolide antibiotic that primarily targets Gram-positive and Gram-negative bacteria, fungi, and some viruses. PCP is a fungal infection, so azithromycin does not effectively target Pneumocystis jiroveci and is less effective compared to TMP-SMX.

C. Fluconazole: Fluconazole is a triazole antifungal agent primarily targeting yeasts, particularly Candida species. It does not have significant activity against Pneumocystis jiroveci, making it an unsuitable choice for treating PCP.

D. Amphotericin B: Amphotericin B is a polyene antifungal agent that binds to ergosterol in fungal cell membranes, causing leakage and cell death. While it has activity against certain fungi, amphotericin B is less effective than TMP-SMX and has more severe side effects, making it an inferior choice for treating PCP compared to TMP-SMX.

**Clinical Pearl:**

Trimethoprim-sulfamethoxazole (TMP-SMX) is the preferred choice for treating Pneumocystis jiroveci pneumonia due to its dual mechanism of action, targeting both bacteria and fungi, and its high efficacy against Pneumocystis jiroveci.