**Core Concept**
The underlying principle here is the diagnosis and treatment of sexually transmitted infections (STIs), specifically urethritis. Urethritis is an inflammation of the urethra, often caused by bacterial or viral infections. The patient's history of multiple sex partners increases the likelihood of an STI.

**Why the Correct Answer is Right**
The patient likely has gonococcal urethritis, caused by Neisseria gonorrhoeae. The best treatment option is dual therapy with **ceftriaxone and azithromycin**. Ceftriaxone is a cephalosporin antibiotic that is effective against N. gonorrhoeae, while azithromycin is a macrolide antibiotic that provides coverage against chlamydia, another common cause of urethritis. This combination is recommended by the CDC for the treatment of gonococcal urethritis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ciprofloxacin is a quinolone antibiotic that was previously used to treat gonococcal infections, but it is no longer recommended due to widespread resistance.
* **Option B:** Penicillin is not effective against N. gonorrhoeae, as this bacterium has become resistant to penicillin.
* **Option D:** Metronidazole is an antibiotic used to treat anaerobic bacterial infections, but it is not effective against N. gonorrhoeae.

**Clinical Pearl / High-Yield Fact**
Always consider dual therapy when treating STIs, as co-infections are common and can lead to treatment failure if not adequately addressed.

**Correct Answer: C. Ceftriaxone and azithromycin.**