**Core Concept**
The question tests the understanding of cross-reactivity between different classes of drugs, specifically the structural similarity between sulphonamides and other antimicrobials. Sulphonamides are a group of antimicrobials that inhibit folic acid synthesis in bacteria.

**Why the Correct Answer is Right**
The correct answer is based on the principle of cross-reactivity, where an allergic reaction to one drug can also occur with structurally similar drugs. Sulphonamides and trimethoprim are closely related in structure and mechanism of action, and an allergy to sulphonamides can also cause an allergic reaction to trimethoprim. This is because both drugs inhibit the same enzyme, dihydrofolate reductase, in bacteria.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a sulphonamide or a closely related drug. It is a different class of antimicrobial that does not have the same cross-reactivity with sulphonamides.
* **Option B:** This option is incorrect because it is not commonly used in clinical practice and is not structurally similar to sulphonamides.
* **Option C:** This option is incorrect because it is a different class of antimicrobial that is not related to sulphonamides in terms of structure or mechanism of action.

**Clinical Pearl / High-Yield Fact**
It is essential to note that patients with allergies to sulphonamides may also experience an allergic reaction to trimethoprim, and vice versa. This highlights the importance of careful drug selection in patients with a history of allergies to certain medications.

**Correct Answer:** C.