**Core Concept**
The patient's symptoms suggest a chronic infection of the paranasal sinuses, specifically the anterior ethmoid, frontal, and maxillary sinuses. This condition is known as chronic rhinosinusitis (CRS), characterized by inflammation of the mucosal lining of the paranasal sinuses.

**Why the Correct Answer is Right**
The patient's symptoms persist despite initial response to antibiotics, indicating a possible anatomical obstruction or an underlying inflammatory process. The congested and oedematous mucosa in the middle meatus suggests involvement of the ostiomeatal complex, a critical area where the frontal, anterior ethmoid, and maxillary sinuses drain. A computed tomography (CT) scan of the paranasal sinuses would be the next best investigation to evaluate the extent of sinus involvement, identify any anatomical obstructions, and guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Endoscopy of the nasal cavity would provide limited information about the sinuses themselves and may not detect underlying anatomical obstructions or inflammatory changes.

**Option B:** MRI of the paranasal sinuses is not typically used as a first-line imaging modality for CRS, as it may not provide sufficient detail of the bony structures and sinuses.

**Option C:** Allergy testing may be relevant in some cases of CRS, but it is not the next best investigation in this patient, who has already shown a response to antibiotics.

**Clinical Pearl / High-Yield Fact**
Chronic rhinosinusitis is a common condition that requires a thorough evaluation of both the nasal cavity and paranasal sinuses, including imaging studies to guide management and exclude other underlying conditions.

**Correct Answer:** C.