**Core Concept**
The primary treatment of syphilis involves the use of antibiotics to target the causative bacterium, Treponema pallidum. The preferred treatment regimen is based on the stage and severity of the disease, as well as potential resistance patterns.

**Why the Correct Answer is Right**
The drug of choice for primary syphilis is penicillin, specifically benzathine penicillin G (BPG) administered intramuscularly. This is because penicillin is highly effective against Treponema pallidum, and BPG provides a high therapeutic index, allowing for a single dose to be effective in treating the infection. The mechanism of action involves the inhibition of bacterial cell wall synthesis, ultimately leading to the death of the bacteria.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because erythromycin is a macrolide antibiotic that is not the first-line treatment for syphilis. While it may be effective, it is not the preferred choice due to potential resistance and side effects.
* **Option B:** This option is incorrect because doxycycline is a tetracycline antibiotic that is used to treat various infections, but it is not the primary treatment for syphilis. It may be used as an alternative in cases of penicillin allergy.
* **Option C:** This option is incorrect because ceftriaxone is a cephalosporin antibiotic that is effective against a range of bacterial infections, but it is not the first-line treatment for syphilis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that penicillin resistance is extremely rare in Treponema pallidum, making it an ideal treatment option. However, patients with a history of penicillin allergy require careful evaluation and alternative treatments may be necessary.

**Correct Answer:** C.