Sinus which is not present at birth is?
**Core Concept**
The question is testing the knowledge of **paranasal sinuses** development. The paranasal sinuses are air-filled spaces around the nasal cavity that develop at different stages of growth. Understanding the timeline of their development is crucial for diagnosing and managing sinus-related conditions.
**Why the Correct Answer is Right**
The **frontal sinus** is the sinus that is not present at birth. It begins to develop around the second year of life from the anterior ethmoidal cells and becomes fully formed by the age of 6-8 years. This delayed development makes it distinct from other sinuses like the maxillary and ethmoid sinuses, which are present at birth.
**Why Each Wrong Option is Incorrect**
**Option A:** The maxillary sinus is indeed present at birth, making it an incorrect choice.
**Option B:** The ethmoid sinuses are also present at birth.
**Option D:** The sphenoid sinus, although small, is present at birth as well.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the development of sinuses can be relevant in diagnosing sinusitis or other conditions in children, as their symptoms and treatment may vary based on the developmental stage of their sinuses.
**Correct Answer:** D. Frontal sinus.