**Core Concept:** Sinusitis refers to inflammation of the paranasal sinuses, which are air-filled cavities in the skull bones. The primary cause is usually viral infections, but bacterial and fungal infections can also lead to sinusitis. Chronic sinusitis refers to inflammation lasting more than 12 weeks, while acute sinusitis is a shorter episode.

**Why the Correct Answer is Right:** Sinusitis is least involved in the **sphenoid sinus** due to its deep location within the skull and protected position. It is also less likely to occur in the **maxillary sinus** because of its thin mucosal lining and close proximity to the oral cavity, which facilitates easy access to pathogens. The **frontal sinus** is rarely affected due to its distance from the nasal cavity and entrance to the respiratory tract.

**Why Each Wrong Option is Incorrect:** Option **A** (frontal sinus) is incorrect because of its distance from the nasal cavity and the respiratory tract, making it less susceptible to infection. Option **B** (ethmoid sinus) is less involved due to its location within the ethmoid bone, which is a dense, compact bone that provides a protective barrier. Option **D** (nasal cavity) is incorrect because sinusitis typically involves the nasal cavity first, facilitating the spread to the paranasal sinuses.

**Clinical Pearl:** Understanding the anatomy of the paranasal sinuses is crucial when assessing patients with sinusitis, as knowing which sinuses are less involved can help narrow down the potential causative organisms and guide appropriate diagnostic and therapeutic strategies.

**Correct Answer:** .