Which antibiotic is the drug of first choice for a chlamydial infection when Gram stain is negative for intracellular diplococci and there are no allergies?

• A. Doxycycline
• B. Penicillin G
• C. Ciprofloxacin
• D. Cephalexin

Answer: (A)

Chlamydia infections require antibiotics that reach and act inside host cells. Doxycycline, a tetracycline, binds the 30S ribosomal subunit and prevents incorporation of aminoacyl-tRNA, stopping protein synthesis. Since Chlamydia trachomatis lives inside cells, doxycycline achieves effective intracellular concentrations and is highly active against it. The standard regimen is 100 mg twice daily for 7 days, offering reliable cure for cervicitis and urethritis. Azithromycin 1 g single-dose is an acceptable alternative if adherence is a concern, but doxycycline is the preferred first choice when there are no contraindications. Penicillin G and cephalexin target cell-wall synthesis and are not reliably effective against chlamydia due to its intracellular lifestyle, while ciprofloxacin can be used in some scenarios but is not the first-line option given current guidelines and resistance considerations.