**Core Concept**
The treatment of Chlamydial infection involves the use of antibiotics that target the intracellular pathogens. Chlamydia trachomatis is an obligate intracellular bacterium that requires specific antibiotics to eliminate the infection.

**Why the Correct Answer is Right**
The treatment of choice for Chlamydial infection is Azithromycin, a macrolide antibiotic that concentrates in the cell, allowing it to effectively target the intracellular pathogens. Azithromycin inhibits protein synthesis by binding to the 50S ribosomal subunit, preventing the translation of bacterial mRNA into proteins. This leads to the death of the Chlamydia trachomatis bacteria. Azithromycin is also effective in treating other sexually transmitted infections such as Mycoplasma genitalium.

**Why Each Wrong Option is Incorrect**
* **Option A:** Doxycycline is a tetracycline antibiotic that is effective against Chlamydia trachomatis, but it is not the first-line treatment due to its longer treatment duration and potential side effects. Doxycycline is often used in combination with Azithromycin to treat Chlamydial infections.
* **Option B:** Ceftriaxone is a cephalosporin antibiotic that is effective against a wide range of bacterial infections, but it is not the treatment of choice for Chlamydial infections. Ceftriaxone is often used to treat gonococcal infections, which can be co-infections with Chlamydia.
* **Option D:** Metronidazole is an antibiotic that is effective against anaerobic bacteria, but it is not effective against Chlamydia trachomatis.

**Clinical Pearl / High-Yield Fact**
Azithromycin is a broad-spectrum antibiotic that is also effective in treating other sexually transmitted infections, including Mycoplasma genitalium. It is essential to consider co-infections when treating Chlamydial infections, and Azithromycin is often used as a first-line treatment due to its effectiveness and convenience.

**Correct Answer:** C. Azithromycin.