### Core Concept
Chlamydia trachomatis is a sexually transmitted bacterium causing infection in pregnant women, potentially leading to adverse pregnancy outcomes. Effective treatment of chlamydia in pregnancy requires a safe and effective antibiotic.

### Why the Correct Answer is Right
Azithromycin is the preferred treatment for chlamydia in pregnancy due to its efficacy, safety profile, and convenience. It is a macrolide antibiotic that inhibits protein synthesis by binding to the bacterial 50S ribosomal subunit, thus preventing peptidyl transferase activity. This results in the disruption of bacterial protein synthesis and ultimately leads to bacterial death. Azithromycin's safety profile in pregnancy has been well-established, making it a suitable choice for treating chlamydia during this period.

### Why Each Wrong Option is Incorrect
**Option A:** Amoxicillin is not the preferred choice for treating chlamydia in pregnancy, as it is not effective against Chlamydia trachomatis and can cause gastrointestinal side effects.
**Option B:** Erythromycin, although a macrolide antibiotic, is not typically used for treating chlamydia in pregnancy due to its potential for causing gastrointestinal side effects and its less favorable pharmacokinetic profile compared to azithromycin.
**Option C:** Ceftriaxone is a cephalosporin antibiotic that is effective against a broad range of bacterial infections, but it is not the preferred choice for treating chlamydia in pregnancy due to its potential for causing allergic reactions and the availability of safer alternatives.

### Clinical Pearl / High-Yield Fact
When treating chlamydia in pregnancy, it's essential to note that a single dose of azithromycin (1 g) is often sufficient for treatment, eliminating the need for a full treatment course.

### Correct Answer: D. Azithromycin