**Core Concept**
Leptospirosis is a bacterial infection caused by Leptospira species, which can lead to severe symptoms such as fever, headache, and kidney or liver failure. Effective treatment involves antibiotics that target the bacterial infection.

**Why the Correct Answer is Right**
Penicillin is the recommended drug for treating leptospirosis due to its bactericidal activity against Leptospira species. It works by inhibiting cell wall synthesis, ultimately leading to the death of the bacteria. The recommended dose is usually intravenous penicillin G, with a duration of treatment ranging from 7 to 14 days.

**Why Each Wrong Option is Incorrect**
**Option B:** Tetracycline is not the first-line treatment for leptospirosis and may not be effective due to resistance.
**Option C:** Azithromycin is a macrolide antibiotic that is not typically used to treat leptospirosis, as it is not bactericidal against Leptospira species.
**Option D:** Erythromycin is another macrolide antibiotic that is not recommended for treating leptospirosis, as it has limited activity against Leptospira.

**Clinical Pearl / High-Yield Fact**
Leptospirosis can be prevented by avoiding exposure to contaminated water or soil, and by using protective clothing and vaccines in high-risk settings, such as occupational and recreational activities.

✓ Correct Answer: A. Penicillin