**Core Concept**
The question tests the knowledge of the treatment of syphilis in pregnancy. Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum, and it can be transmitted from mother to fetus during pregnancy. The standard treatment for syphilis in pregnancy involves administering antibiotics to prevent congenital syphilis.

**Why the Correct Answer is Right**
The Centers for Disease Control and Prevention (CDC) recommends benzathine penicillin G as the first-line treatment for syphilis in pregnancy. This is because benzathine penicillin G is effective in treating syphilis and preventing congenital syphilis, and it has a long duration of action, reducing the need for frequent injections. Benzathine penicillin G works by inhibiting bacterial cell wall synthesis, ultimately leading to the death of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Doxycycline is contraindicated in pregnancy due to the risk of inhibiting bone growth and causing tooth discoloration in the fetus.
**Option B:** Ceftriaxone has been used as an alternative treatment for syphilis in pregnancy, but it is not the first-line treatment. Additionally, it requires more frequent injections compared to benzathine penicillin G.
**Option C:** Macrolides such as azithromycin have been used as an alternative treatment for syphilis in pregnancy, but they are not the first-line treatment and may have a higher failure rate compared to benzathine penicillin G.

**Clinical Pearl / High-Yield Fact**
It's essential to note that all pregnant women with syphilis should be treated with benzathine penicillin G, regardless of their stage of pregnancy or symptoms. This is crucial to prevent congenital syphilis and ensure a healthy outcome for the mother and the fetus.

**Correct Answer: C. Ceftriaxone has been used as an alternative treatment for syphilis in pregnancy, but it is not the first-line treatment. Additionally, it requires more frequent injections compared to benzathine penicillin G.