**Core Concept**
Leptospirosis is a zoonotic infection caused by the bacterium *Leptospira*, which enters the body through contact with contaminated water or soil. The disease can range from a mild flu-like illness to severe complications such as meningitis, respiratory distress, and renal failure.

**Why the Correct Answer is Right**
The drug of choice for treating leptospirosis is penicillin, specifically intravenous penicillin G. This is because *Leptospira* is a gram-negative bacterium that is sensitive to penicillin. Penicillin works by inhibiting cell wall synthesis, ultimately leading to bacterial lysis and death. The recommended dose of penicillin G for leptospirosis is 2-4 million units intravenously every 4-6 hours for 7-14 days.

**Why Each Wrong Option is Incorrect**
**Option A:** Doxycycline is an antibiotic that is often used to treat other bacterial infections, but it is not the first-line treatment for leptospirosis.
**Option B:** Ceftriaxone is a cephalosporin antibiotic that may be effective against some gram-negative bacteria, but it is not the preferred treatment for leptospirosis.
**Option C:** Azithromycin is a macrolide antibiotic that is used to treat a variety of bacterial infections, but it is not effective against *Leptospira*.

**Clinical Pearl / High-Yield Fact**
It's essential to note that early recognition and treatment of leptospirosis can significantly improve outcomes. Patients with symptoms such as fever, headache, and muscle aches should be considered for leptospirosis testing, especially if they have a history of exposure to contaminated water or soil.

**Correct Answer:** D. Penicillin G.