Step 1: What is Cotrimoxazole?
Cotrimoxazole is a fixed-dose combination antibiotic widely used to treat urinary tract infections, respiratory infections, and Pneumocystis jirovecii pneumonia. The name itself is a clue: it comes from Co (combination) + tri (trimethoprim) + moxazole (sulphamethoxazole).
Step 2: Identify the two components.
The combination is Sulphamethoxazole + Trimethoprim, typically in a 5:1 ratio (400 mg sulphamethoxazole : 80 mg trimethoprim in a standard tablet). These two drugs work synergistically on sequential steps of the folate synthesis pathway in bacteria.
Step 3: Why this specific combination?
Sulphamethoxazole inhibits dihydropteroate synthase (earlier step). Trimethoprim inhibits dihydrofolate reductase (later step). Blocking two sequential steps gives better bactericidal effect and reduces resistance.
Step 4: Rule out the distractors.
Option 2 says Sulfadoxine + Trimethoprim: Sulfadoxine is used with pyrimethamine for malaria, not with trimethoprim in Cotrimoxazole. Option 3 says Sulfadiazine + Trimethoprim: Sulfadiazine is used for toxoplasmosis. Option 4 says Sulfasalazine + Trimethoprim: Sulfasalazine is used for inflammatory bowel disease. None of these is Cotrimoxazole.
Step 5: Confirm the answer.
Cotrimoxazole = Sulphamethoxazole + Trimethoprim. This is a fundamental pharmacology fact.
Answer: Option (1) — Sulphamethoxazole + Trimethoprim