**Core Concept**
Co-trimoxazole is a combination antibiotic consisting of trimethoprim and sulfamethoxazole, used to treat various bacterial infections. The ratio of trimethoprim to sulfamethoxazole in co-trimoxazole is crucial for its effectiveness and to minimize the development of resistance.

**Why the Correct Answer is Right**
The optimal ratio of trimethoprim to sulfamethoxazole is 1:5 to 1:20. This ratio allows for the synergistic inhibition of bacterial dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) enzymes, which are essential for bacterial DNA synthesis and growth. Trimethoprim inhibits DHFR, while sulfamethoxazole inhibits DHPS.

**Why Each Wrong Option is Incorrect**
**Option A:** This ratio is too low, and it would not effectively inhibit bacterial growth.
**Option B:** This ratio is too high, and it may lead to an increased risk of trimethoprim toxicity.
**Option C:** This ratio is incorrect, and it would not provide the synergistic effect required for effective antibacterial activity.

**Clinical Pearl / High-Yield Fact**
Remember that the ratio of trimethoprim to sulfamethoxazole is critical for the effectiveness of co-trimoxazole. A 1:5 to 1:20 ratio is generally recommended to minimize the development of resistance and ensure optimal therapeutic effects.

**Correct Answer: C. 1:5 to 1:20**