Bactrim is a combination of which two agents?

• A. Sulfamethoxazole and Trimethoprim
• B. Ampicillin and Clavulanate
• C. Ciprofloxacin and Doxycycline
• D. Erythromycin and Azithromycin

Answer: (A)

Bactrim is a fixed-dose combination that pairs sulfamethoxazole with trimethoprim. These two drugs work together by blocking two sequential steps in bacterial folate synthesis, which is essential for making DNA and other nucleotides. Sulfamethoxazole inhibits dihydropteroate synthase, while trimethoprim inhibits dihydrofolate reductase. Because each drug blocks a different step in the same pathway, their effects add up to a stronger, bactericidal impact than either would alone. This synergy is why the combination is marketed as Bactrim (also known as co-trimoxazole).

The other pairs are different kinds of antibiotics or are not marketed as a single, fixed combination called Bactrim. Ampicillin with clavulanate is a beta-lactam antibiotic paired with a beta-lactamase inhibitor (not a folate synthesis inhibitor). Ciprofloxacin with doxycycline are two separate antibiotics used in various regimens but not a standard Bactrim product. Erythromycin and azithromycin are both macrolides, not a two-drug fixed combination like this.