**Core Concept:**
Sphenoid sinusitis is inflammation of the sphenoid sinus, a paired paranasal sinus located within the skull behind the nasal cavity. The sphenoid sinus communicates with the nasal cavity via the sphenopalatine foramen and with the cavernous sinus via the greater sphenoid sinus ostium. Pain referral patterns in sinusitis are an important aspect of clinical diagnosis.

**Why the Correct Answer is Right:**
The correct answer, option D (V3), refers to the trigeminal nerve (V3 branch), which is responsible for sensory innervation of the posterior aspect of the face, including the temporal region, mastoid process, and the sphenoid sinus. When the sphenoid sinus becomes inflamed (sphenoid sinusitis), the pain is referred to the V3 distribution due to its proximity and the neuroanatomical pathway.

**Why Each Wrong Option is Incorrect:**

A. Refers to the mandibular nerve (V2), which is responsible for sensory innervation of the lower lip, chin, and mandible. Since sphenoid sinusitis does not involve the lower face, this option is incorrect.

B. Refers to the maxillary nerve (V2), which innervates the maxilla, nasal mucosa, and teeth. Sphenoid sinusitis does not cause pain in these areas, rendering this option incorrect.

C. Refers to the ophthalmic nerve (V1), which innervates the forehead, eye, and eyelid. Sphenoid sinusitis does not cause pain in these areas, making option C incorrect.

D. Refers to the trigeminal nerve (V3), as explained above. This option is the correct answer because sphenoid sinusitis causes referred pain to the posterior aspect of the face, which is innervated by the V3 branch.

**Clinical Pearl:**
The understanding of pain referral patterns is essential in differentiating between primary (local) facial pain and referred pain. In the case of sphenoid sinusitis, recognizing the correct trigeminal nerve (V3) referral pattern helps in narrowing down the differential diagnosis and guiding further diagnostic and therapeutic steps.