**Core Concept**
Cotrimoxazole is a combination antibiotic that consists of two active components: sulphamethoxazole (a sulphonamide) and trimethoprim (a dihydrofolate reductase inhibitor). The synergistic effect of these two drugs is crucial for their antimicrobial activity.

**Why the Correct Answer is Right**
The ratio of sulphamethoxazole to trimethoprim in cotrimoxazole is 5:1. This ratio is essential for achieving the optimal synergistic effect, as sulphamethoxazole is a weak inhibitor of dihydropteroate synthase, while trimethoprim is a potent inhibitor of dihydrofolate reductase. The 5:1 ratio ensures that trimethoprim is present in sufficient concentrations to inhibit dihydrofolate reductase effectively, while sulphamethoxazole provides a broad spectrum of activity against various bacteria. This synergistic combination is responsible for the antimicrobial efficacy of cotrimoxazole.

**Why Each Wrong Option is Incorrect**
**Option A:** The ratio of 2:1 is not sufficient to achieve the optimal synergistic effect between sulphamethoxazole and trimethoprim.
**Option B:** A 1:1 ratio would not provide the necessary balance between the two components to inhibit bacterial growth effectively.
**Option D:** The ratio of 4:5 is incorrect and would not be effective in achieving the desired synergistic effect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the 5:1 ratio of sulphamethoxazole to trimethoprim in cotrimoxazole is crucial for its efficacy. This ratio should be maintained when prescribing cotrimoxazole to ensure optimal antimicrobial activity.

**✓ Correct Answer: C. 5:01**