**Core Concept**
The frontal and ethmoidal sinuses are the most commonly affected sinuses in adults, but in children, the paranasal sinuses are not fully developed. The maxillary and sphenoid sinuses are present at birth, but the frontal and ethmoidal sinuses develop later, often not reaching full size until the second decade of life.

**Why the Correct Answer is Right**
In children, the paranasal sinuses are smaller and more prone to infection due to their narrow drainage pathways and the presence of cilia that are not yet fully developed. The maxillary sinus is the largest of the paranasal sinuses and is located in the maxillary bone, which is a common site for sinusitis in children. The maxillary sinus has a relatively short drainage pathway and is more susceptible to bacterial and viral infections.

**Why Each Wrong Option is Incorrect**
**Option A:** The frontal sinus is not fully developed in children and is not typically affected in cases of pediatric sinusitis.
**Option B:** The sphenoid sinus is relatively small in children and is not commonly affected in cases of sinusitis.
**Option C:** The ethmoidal sinuses are present at birth, but they are not the most commonly affected sinuses in children.

**Clinical Pearl / High-Yield Fact**
The maxillary sinus is the most commonly affected sinus in children due to its relatively short drainage pathway and the presence of cilia that are not yet fully developed. This is an important consideration for pediatricians and otolaryngologists when diagnosing and treating sinusitis in children.

**Correct Answer:** A.