Which of the following is a sulfonamide medication?

The correct choice is Trimethoprim/sulfamethoxazole, as it is a combination antibiotic that includes sulfamethoxazole, which is a sulfonamide. Sulfonamides are a group of synthetic antibiotics that inhibit bacterial growth by interfering with the production of folic acid. This action is crucial because folic acid is necessary for the synthesis of nucleic acids in bacteria, thus hindering their ability to reproduce and spread.

Trimethoprim enhances the effectiveness of sulfamethoxazole by inhibiting a different step in the bacterial folic acid synthesis pathway. Together, they provide a synergistic effect, making this combination highly effective against a variety of infections, including urinary tract infections and some respiratory tract infections.

In contrast, Amoxicillin, Cephalexin, and Azithromycin belong to different classes of antibiotics. Amoxicillin is a penicillin-type antibiotic, Cephalexin is a cephalosporin, and Azithromycin is a macrolide. Each of these medications works through different mechanisms and does not contain the sulfonamide structure or function in the same way as the combination of Trimethoprim and sulfamethoxazole.