**Core Concept**
Stenotrophomonas maltophilia is a Gram-negative, opportunistic pathogen that commonly causes infections in immunocompromised patients, particularly in those with cystic fibrosis, HIV/AIDS, or cancer. Effective treatment requires antibiotics with broad-spectrum activity.

**Why the Correct Answer is Right**
The antibiotic of choice for Stenotrophomonas maltophilia infection is trimethoprim-sulfamethoxazole (TMP-SMX). This combination works by inhibiting dihydrofolate reductase (DHFR) in the bacterial enzyme pathway, thereby preventing tetrahydrofolate synthesis. This is crucial for bacterial DNA synthesis and replication. TMP-SMX has been shown to be effective in treating S. maltophilia infections, including bacteremia, pneumonia, and urinary tract infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because ciprofloxacin, a fluoroquinolone antibiotic, may exhibit in vitro activity against S. maltophilia but is not the preferred treatment due to concerns of resistance development and variable efficacy in clinical settings.
* **Option B:** This option is incorrect because amoxicillin-clavulanate is a broad-spectrum antibiotic combination, but it lacks efficacy against S. maltophilia, which is inherently resistant to many beta-lactam antibiotics.
* **Option D:** This option is incorrect because gentamicin, an aminoglycoside antibiotic, is not effective against S. maltophilia due to its inherent resistance, and its use may be associated with nephrotoxicity and ototoxicity.

**Clinical Pearl / High-Yield Fact**
TMP-SMX is also effective against other Gram-negative pathogens, such as Pseudomonas aeruginosa and Escherichia coli, making it a valuable option in treating complex infections.

**Correct Answer:** C. Trimethoprim-sulfamethoxazole.