Question

Blood concentration ratio of the combination of trimethoprim and sulfamethoxazole after being given in a 1:5 ratio is

• A. 1:1
• B. 1:5
• C. 1:10
• D. 1:20

Hint:

Trimethoprim inhibits dihydrofolate reductase, while sulfamethoxazole inhibits dihydropteroate synthase—together they block folate synthesis, leading to synergistic action.

Answer 1

The Correct Option is D

Trimethoprim and sulfamethoxazole are dosed together at a 1:5 ratio, but that is not the same as the ratio their concentrations reach in the bloodstream once absorbed.

1. 1:1: Would require identical distribution and clearance for both drugs, which is not the case here.
2. 1:5: This is the dose ratio in the tablet formulation itself, not the resulting blood concentration ratio.
3. 1:10: Does not correspond to the documented blood level ratio for this combination.
4. 1:20: Sulfamethoxazole's faster clearance and larger volume of distribution compared to trimethoprim widen the gap between the two drugs once they are in the bloodstream, landing close to a 1:20 ratio, which happens to match the ratio found to work best synergistically against bacteria.

The dose ratio of 1:5 was chosen specifically to produce a blood ratio near 1:20.

So the correct answer is 1:20.