Sinus not present at bih is:
**Core Concept**
The question is testing the knowledge of the development and anatomy of the paranasal sinuses in newborns. The paranasal sinuses are air-filled cavities that develop from outgrowths of the nasal cavity and are present at birth, but some of them are not fully developed or pneumatized.
**Why the Correct Answer is Right**
The maxillary sinus is the largest of the paranasal sinuses and is present at birth, but it is not fully developed or pneumatized until around 2-3 years of age. The frontal, sphenoid, and ethmoid sinuses are also present at birth, but they are not fully developed until later in childhood. The mastoid air cells, which are part of the temporal bone, are also not fully pneumatized until around 2-3 years of age.
**Why Each Wrong Option is Incorrect**
**Option A:** The mastoid air cells are not present at birth, but rather develop later in childhood.
**Option B:** The sphenoid sinus is present at birth, but is not fully developed until later in childhood.
**Option C:** The ethmoid sinus is present at birth, but is not fully developed until later in childhood.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the paranasal sinuses are not fully developed until later in childhood, which can lead to complications such as sinusitis or other respiratory issues if they become infected.
**Correct Answer:** C. Ethmoid sinus.