**Core Concept**
Syphilis during pregnancy poses a significant risk to both the mother and the fetus, making it essential to select an effective and safe treatment option.

**Why the Correct Answer is Right**
The drug of choice for syphilis during pregnancy is **Penicillin**, specifically **benzathine penicillin G** (Bicillin L-A). This is because penicillin is the only antibiotic that has been consistently shown to be effective in treating syphilis in pregnant women. Penicillin works by inhibiting bacterial cell wall synthesis, ultimately leading to the death of the bacteria. The recommended dose for pregnant women is usually 2.4 million units administered intramuscularly in a single dose.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because other antibiotics like erythromycin and tetracycline are not recommended for treating syphilis in pregnant women due to their potential side effects and the risk of resistance development.
**Option B:** This option is incorrect because ceftriaxone, a cephalosporin antibiotic, is not the preferred treatment for syphilis in pregnancy, although it may be used in cases of penicillin allergy.
**Option C:** This option is incorrect because azithromycin, a macrolide antibiotic, is not the first-line treatment for syphilis in pregnant women, although it may be used in cases of penicillin allergy.

**Clinical Pearl / High-Yield Fact**
In cases of penicillin allergy, an alternative treatment option for syphilis in pregnant women is **ceftriaxone**, but this should be used under close supervision and with proper monitoring.

**Correct Answer: D. Penicillin (specifically benzathine penicillin G)**