Question:
What is Cotrimoxazole?
What is Cotrimoxazole?
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Cotrimoxazole is a well-known fixed-dose combination antibiotic. Both of its ingredients block the same bacterial pathway (folic acid synthesis) at two different steps, so together they act more powerfully.
Updated On: Jun 24, 2026
- Sulphamethoxazole + Trimethoprim
- Sulfadoxine + Trimethoprim
- Sulfadiazine + Trimethoprim
- Sulfasalazine + Trimethoprim
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The Correct Option is A
Solution and Explanation
Concept:
Cotrimoxazole is a well-known fixed-dose combination antibiotic. Both of its ingredients block the same bacterial pathway (folic acid synthesis) at two different steps, so together they act more powerfully.
Step 1: The first component is the sulphonamide sulphamethoxazole, which blocks the enzyme dihydropteroate synthase.
Step 2: The second component is trimethoprim, which blocks the next enzyme, dihydrofolate reductase. Hitting two consecutive steps gives a synergistic, often bactericidal effect.
Why the others are wrong: Sulfadoxine is paired with pyrimethamine (an antimalarial combination), not in cotrimoxazole. Sulfadiazine and sulfasalazine are used for other indications and are not the sulphonamide in cotrimoxazole. The correct pair is sulphamethoxazole plus trimethoprim, usually in a 5:1 ratio.
Answer: Option (1) — Sulphamethoxazole + Trimethoprim.
Cotrimoxazole is a well-known fixed-dose combination antibiotic. Both of its ingredients block the same bacterial pathway (folic acid synthesis) at two different steps, so together they act more powerfully.
Step 1: The first component is the sulphonamide sulphamethoxazole, which blocks the enzyme dihydropteroate synthase.
Step 2: The second component is trimethoprim, which blocks the next enzyme, dihydrofolate reductase. Hitting two consecutive steps gives a synergistic, often bactericidal effect.
Why the others are wrong: Sulfadoxine is paired with pyrimethamine (an antimalarial combination), not in cotrimoxazole. Sulfadiazine and sulfasalazine are used for other indications and are not the sulphonamide in cotrimoxazole. The correct pair is sulphamethoxazole plus trimethoprim, usually in a 5:1 ratio.
Answer: Option (1) — Sulphamethoxazole + Trimethoprim.
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