**Core Concept**
Pneumocystis carinii (now known as Pneumocystis jirovecii) pneumonia (PCP) is a life-threatening opportunistic infection primarily affecting immunocompromised individuals, such as those with HIV/AIDS. The pathogen is a fungus that colonizes the lungs, causing severe inflammation and damage.

**Why the Correct Answer is Right**
Cotrimoxazole (trimethoprim-sulfamethoxazole, TMP-SMX) is the first-line treatment for PCP due to its potent antimicrobial activity against P. jirovecii. Trimethoprim inhibits dihydrofolate reductase, an enzyme essential for the synthesis of tetrahydrofolate, while sulfamethoxazole competes with para-aminobenzoic acid (PABA) for binding to dihydropteroate synthase, another enzyme crucial for folate synthesis. This dual-action mechanism effectively prevents the growth of P. jirovecii, allowing the host's immune system to recover.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillin is ineffective against P. jirovecii as it is a fungus, not a bacterium. Penicillin targets bacterial cell walls by inhibiting cell wall synthesis, which is irrelevant in fungal infections.
**Option C:** Kanamycin is an aminoglycoside antibiotic primarily used to treat gram-negative bacterial infections. It is not effective against P. jirovecii.
**Option D:** Levofloxacin is a fluoroquinolone antibiotic with broad-spectrum activity against gram-positive and gram-negative bacteria. However, it is not the preferred treatment for PCP due to concerns about resistance development and the availability of more effective options.

**Clinical Pearl / High-Yield Fact**
In patients with PCP, cotrimoxazole should be administered for at least 21 days, followed by secondary prophylaxis to prevent recurrence. This prolonged treatment duration and prophylaxis regimen are crucial to manage the infection effectively.

**✓ Correct Answer: B. Cotrimoxazole**