## **Core Concept**
The question tests knowledge of sulfonamides, a class of antimicrobials that inhibit folic acid synthesis in bacteria. Sulfonamides vary in their pharmacokinetic properties, including their duration of action, which is largely determined by their elimination half-life and protein binding.

## **Why the Correct Answer is Right**
Sulphadiazine, Sulphamethoxazole, Sulphadimidine, and Sulphamethoxazole-Trimethoprim are commonly known sulfonamides. Among these, **Sulphamethoxazole** has a notably long half-life, which allows for less frequent dosing. This property makes it particularly useful in combination with trimethoprim (as co-trimoxazole) for treating various bacterial infections.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While specific details about option A are not provided, if it is not Sulphamethoxazole, it likely refers to another sulfonamide with a shorter half-life or less notable characteristics.
- **Option B:** Similarly, without specifics, if option B is incorrect, it might represent a sulfonamide with a shorter duration of action or less commonly referenced in this context.
- **Option D:** This option, being incorrect, likely represents another antimicrobial or a sulfonamide with a shorter half-life compared to Sulphamethoxazole.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Sulphamethoxazole**, often in combination with trimethoprim (co-trimoxazole), is effective against a wide range of bacterial infections and has a long half-life, making it suitable for twice-daily dosing. This combination is particularly noted for its efficacy against *Pneumocystis jirovecii* pneumonia.

## **Correct Answer:** C. Sulphamethoxazole.