**Core Concept**
The ratio of concentration of drug in co-trimoxazole of trimethoprim and sulfamethoxazole is crucial in the treatment of bacterial infections. Co-trimoxazole is a combination antibiotic consisting of sulfonamide (sulfamethoxazole) and dihydrofolate reductase inhibitor (trimethoprim). The ratio of their concentrations is essential for optimal synergistic effects.

**Why the Correct Answer is Right**
The synergistic effect of co-trimoxazole is achieved when the ratio of trimethoprim to sulfamethoxazole is 1:5 to 1:20. This ratio allows trimethoprim to inhibit dihydrofolate reductase, while sulfamethoxazole inhibits dihydropteroate synthetase. The inhibition of these enzymes disrupts the synthesis of tetrahydrofolate, which is essential for bacterial DNA synthesis and replication. The correct ratio of 1:5 is often used for the treatment of urinary tract infections, while the 1:20 ratio is used for more severe infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** The ratio of 1:1 would not allow for the synergistic effect of co-trimoxazole, as trimethoprim and sulfamethoxazole would inhibit the same enzyme at the same rate.
* **Option B:** The ratio of 1:100 would be too high, leading to an increased risk of trimethoprim toxicity.

**Clinical Pearl / High-Yield Fact**
When prescribing co-trimoxazole, remember that the ratio of trimethoprim to sulfamethoxazole is crucial for optimal efficacy and safety. The correct ratio should be used based on the severity and type of infection.

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