## **Core Concept**
The question tests knowledge of the paranasal sinuses and their involvement in sinusitis. The paranasal sinuses are air-filled cavities around the nasal cavity, and their mucosal lining can become inflamed, leading to sinusitis. The sinuses include the maxillary, ethmoid, frontal, and sphenoid sinuses.

## **Why the Correct Answer is Right**
The sphenoid sinus is often considered the least involved in sinusitis. This is due to several factors: its relatively isolated location, less direct communication with the nasal cavity compared to other sinuses, and possibly less exposure to infectious agents. The sphenoid sinus drains into the sphenoethmoidal recess, which is higher up and more remote from the main nasal airway compared to the drainage pathways of other sinuses.

## **Why Each Wrong Option is Incorrect**
* **Option A: Maxillary sinus** - This sinus is one of the most commonly involved in sinusitis, likely due to its large size, direct communication with the nasal cavity through the semilunar hiatus, and gravity-dependent drainage.
* **Option B: Ethmoid sinus** - The ethmoid sinuses are frequently involved in sinusitis, especially given their complex anatomy with multiple small cells, which can easily become obstructed.
* **Option C: Frontal sinus** - The frontal sinus can also be commonly involved, particularly in the context of anterior ethmoid sinusitis, as the frontal recess (its drainage pathway) is often affected in ethmoid disease.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **sphenoid sinus** is sometimes referred to as the "forgotten sinus" due to its less common involvement in sinusitis and its potential for being overlooked in diagnostic workups. However, its involvement can lead to serious complications due to its proximity to critical structures like the optic nerve and carotid artery.

## **Correct Answer: D. Sphenoid sinus**