Paper II — 2025 March (Supplementary) (2010 Scheme) — Q9

Paper II

2025 March (Supplementary) (2010 Scheme)

Question

Combination of Trimethoprim and Sulfamethoxazole.

Q91 marksShort Answers★✓

Answer

Trimethoprim and sulfamethoxazole together cause sequential blockade of bacterial folate synthesis: sulfamethoxazole competitively inhibits dihydropteroate synthase (an earlier step, incorporation of PABA into folate synthesis), while trimethoprim inhibits dihydrofolate reductase (a later step, reduction of dihydrofolate to active tetrahydrofolate). Blocking two sequential steps in the same pathway simultaneously produces a synergistic, bactericidal effect greater than either drug alone (each individually only bacteriostatic), and reduces the likelihood of resistance emerging compared to either single agent.

Related Concepts

1. 1Sulfonamides and Cotrimoxazole

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