Paper II — 2024 August (Supplementary) (2010 Scheme) — Q8

Paper II

2024 August (Supplementary) (2010 Scheme)

Question

Trimethoprim and Sulfamethoxazole as a combination

Q81 marksShort Answers★✓

Answer

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

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1. 1Sulfonamides and Cotrimoxazole

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