## **Core Concept**
The paranasal sinuses are air-filled cavities surrounding the nasal cavity. At birth, some of these sinuses are already present and begin to pneumatize, while others develop later in childhood. The **ethmoidal sinuses** are known to be present at birth.

## **Why the Correct Answer is Right**
The ethmoidal sinuses are a group of small air cells within the ethmoid bone. They are responsible for drainage and ventilation of the nasal cavity. These sinuses start developing and are present at birth, making them the correct answer. The ethmoidal sinuses continue to develop and increase in number throughout childhood.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **maxillary sinuses** are indeed present at birth but are not as developed. However, they are not the best answer here because ethmoidal sinuses are specifically noted for being present and pneumatized at birth.
- **Option B:** The **sphenoidal sinuses** are not present at birth; they develop much later, usually becoming recognizable on X-rays around the age of 3-5 years.
- **Option D:** The **frontal sinuses** are typically not present at birth; they begin to develop around the age of 6 and become more evident on X-rays around the age of 6-7 years.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the ethmoidal sinuses are present at birth and are a common site for sinusitis, especially in children. Their complex anatomy with multiple small cells makes them prone to infections.

## **Correct Answer:** .