**Core Concept**
Chlamydia trachomatis is a sexually transmitted bacterium causing pelvic inflammatory disease, infertility, and ectopic pregnancy. In pregnancy, Chlamydia can lead to preterm labor, fetal membrane rupture, and neonatal conjunctivitis. Effective treatment of Chlamydia during pregnancy is crucial to prevent maternal and fetal complications.

**Why the Correct Answer is Right**
Erythromycin is the preferred choice for treating Chlamydia infections in pregnant women due to its safety profile and efficacy. This macrolide antibiotic inhibits protein synthesis by binding to the 50S ribosomal subunit of bacteria, ultimately leading to bacterial death. Erythromycin is effective against Chlamydia and is the recommended treatment during pregnancy, as it does not cross the placenta in significant amounts.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetracycline is contraindicated in pregnancy due to its ability to inhibit bone growth and cause tooth discoloration in the fetus.
**Option B:** Doxycycline, a tetracycline derivative, also carries the same risks as tetracycline and is not recommended for use in pregnancy.
**Option D:** Penicillin is not effective against Chlamydia, as this bacterium lacks a cell wall, making beta-lactam antibiotics (such as penicillin) ineffective.

**Clinical Pearl / High-Yield Fact**
When treating Chlamydia in pregnancy, it is essential to ensure that the partner is also treated to prevent re-infection and the potential for adverse outcomes.

✓ Correct Answer: C. Erythromycin