**Core Concept**
Fibrous dysplasia is a benign bone disorder characterized by the replacement of normal bone tissue with fibrous connective tissue and immature bone. This leads to the weakening of the affected bone, making it more susceptible to fracture. In the context of paranasal sinuses, fibrous dysplasia can cause expansion and thinning of the bone, potentially leading to sinusitis and other complications.

**Why the Correct Answer is Right**
The maxillary sinus is the most common paranasal sinus involved by fibrous dysplasia due to its size and location. The maxillary sinus is the largest of the paranasal sinuses and is situated in the maxillary bone, which is a common site for fibrous dysplasia. The expansion of fibrous dysplasia in the maxillary bone can cause thinning of the bone and lead to a higher risk of sinusitis, orbital complications, and other serious conditions.

**Why Each Wrong Option is Incorrect**
**Option B:** The frontal sinus is less commonly affected by fibrous dysplasia due to its smaller size and location in the frontal bone, which is less prone to fibrous dysplasia.
**Option C:** The ethmoid sinus is also a less common site for fibrous dysplasia, likely due to its complex anatomy and the fact that it is composed of multiple small air cells that are less likely to be affected.
**Option D:** The sphenoid sinus is the least common paranasal sinus involved by fibrous dysplasia, possibly due to its deep location in the sphenoid bone, which is less commonly affected by this condition.

**Clinical Pearl / High-Yield Fact**
Fibrous dysplasia can cause expansion and thinning of the bone, potentially leading to sinusitis, orbital complications, and other serious conditions. It is essential to consider fibrous dysplasia in the differential diagnosis of patients with paranasal sinusitis or other complications of the paranasal sinuses.

✓ Correct Answer: A. Maxillary sinus