**Core Concept**
*Treponema pallidum*, the spirochete causing syphilis, is highly susceptible to penicillin due to its cell wall structure and the mechanism of action of penicillin G, which inhibits peptidoglycan synthesis in bacterial cell walls.

**Why the Correct Answer is Right**
Penicillin G is the drug of choice for all stages of syphilis because it effectively penetrates tissues and has high bactericidal activity against *Treponema pallidum*. It works by binding to penicillin-binding proteins, disrupting peptidoglycan synthesis, leading to bacterial lysis. The World Health Organization (WHO) and CDC recommend penicillin G for primary, secondary, latent, and tertiary syphilis due to its superior efficacy and low resistance rates.

**Why Each Wrong Option is Incorrect**
Option B: Tetracycline is effective against *Treponema pallidum* but is not the first-line treatment. It is used only in penicillin-allergic patients and has limited tissue penetration and higher risk of side effects.
Option C: Azithromycin is not effective against *Treponema pallidum*; it is a macrolide with poor activity against spirochetes.
Option D: Doxycycline is used in penicillin-allergic patients, but it is less effective than penicillin G and not the preferred agent.

**Clinical Pearl / High-Yield Fact**
Penicillin G is the gold standard for syphilis treatment—**all stages**—and **no alternative is superior** in efficacy or safety. Always start with penicillin G; reserve alternatives only for true allergies.

✓ Correct Answer: A. Penicillin G