**Core Concept:** Acute sinusitis refers to an inflammation of the paranasal sinuses, typically caused by bacterial, viral, or fungal infections. These infections lead to mucosal edema, inflammation, and obstruction of sinus ostia, resulting in sinus pain, purulent discharge, and systemic symptoms. The most common sinuses affected are those of the ethmoid and maxillary complex.

**Why the Correct Answer is Right:** The correct answer, "D. Maxillary sinus," is chosen due to its anatomical location and extensive drainage area. The maxillary sinus is the largest paranasal sinus and has multiple ostia leading to the nasal cavity, oral cavity, and nasopharynx, making it vulnerable to infections from these regions. Furthermore, its close proximity to the teeth, cheek, and mandible increases the chances of dental and facial pain.

**Why Each Wrong Option is Incorrect:**

A. Frontal sinus: Although involved in severe cases, the frontal sinus is less commonly affected due to its deep location and smaller ostia.
B. Sphenoid sinus: Similar to the frontal sinus, the sphenoid sinus is less commonly affected due to its deep location and smaller ostia.
C. Ethmoid sinus: Although ethmoid sinusitis can present with orbital symptoms, it is less commonly involved due to its complex and narrow drainage system.

**Clinical Pearl:** Understanding the anatomy and drainage pathways of paranasal sinuses is crucial for making a proper diagnosis and selecting appropriate treatment options for sinusitis patients. Ostia drainage patterns help differentiate between ethmoid, frontal, sphenoid, and maxillary sinusitis, guiding the choice of antibiotics and targeted sinus irrigation.

**Correct Answer:** D. Maxillary sinus