**Core Concept:** A mucocele is a cystic swelling caused by the accumulation of mucus in the mucous membrane of a sinus. These swellings are commonly seen in the areas of the paranasal sinuses where the mucous membrane is thin and easily perforated.

**Why the Correct Answer is Right:** Mucoceles are more commonly seen in the paranasal sinuses, specifically the maxillary sinus, due to the thin and easily perforated mucosa of this region. The maxillary sinus is the largest paranasal sinus and has a thin mucous membrane lining, making it more susceptible to mucus accumulation and subsequent mucocele formation.

**Why Each Wrong Option is Incorrect:**

A. Nasal sinus (olfactory sinus): Although mucoceles can occur in the nasal sinus, it is less common compared to the maxillary sinus due to its thick mucous membrane.
B. Ethmoidal sinus: While mucoceles can occur in the ethmoidal sinus, it is less common compared to the maxillary sinus due to the thin mucous membrane in the maxillary sinus.
C. Sphenoidal sinus: Mucoceles in the sphenoidal sinus are relatively rare due to the relatively thick mucous membrane.
D. Sphenopalatine sinus: Mucoceles in this sinus are rare due to the thick mucous membrane.

**Clinical Pearl:** Mucoceles in the maxillary sinus should be differentiated from other cystic swellings in the region, such as a mucopyocele, which is caused by sinusitis and is often associated with sinus pain and purulent discharge.

**Correct Answer:** C. Sphenoidal sinus