**Core Concept**
Herpes zoster ophthalmicus (HZO) is an ocular manifestation of varicella-zoster virus (VZV) reactivation in the trigeminal nerve, specifically involving the ophthalmic division (V1). This condition often presents with a painful rash and vesicles in the distribution of the affected nerve.

**Why the Correct Answer is Right**
The ophthalmic division of the trigeminal nerve (V1) is the most commonly involved nerve in HZO due to its predilection for reactivation in the elderly population. The V1 division supplies the skin and mucous membranes of the forehead, eye, and upper eyelid. Reactivation of VZV in this area leads to inflammation and necrosis of the nerve fibers, resulting in the characteristic rash and pain associated with HZO. The ophthalmic nerve is particularly susceptible to VZV reactivation due to its involvement in the viral latency and its proximity to the eye, which is a common site for VZV reactivation.

**Why Each Wrong Option is Incorrect**

**Option A:** The maxillary division of the trigeminal nerve (V2) is not typically involved in HZO, as it supplies the skin and mucous membranes of the mid-face, which is less commonly affected by VZV reactivation.

**Option B:** The mandibular division of the trigeminal nerve (V3) is not commonly involved in HZO, as it supplies the skin and mucous membranes of the lower face and jaw, which are not typically affected by VZV reactivation.

**Option D:** The facial nerve is not involved in HZO, as it is a separate cranial nerve that supplies the muscles of facial expression and is not associated with VZV reactivation.

**Clinical Pearl / High-Yield Fact**
Herpes zoster ophthalmicus can lead to serious complications, including corneal ulcers, vision loss, and secondary glaucoma, highlighting the importance of prompt diagnosis and treatment to prevent long-term sequelae.

**Correct Answer:** C. The ophthalmic division of the trigeminal nerve (V1) is the most commonly involved nerve in herpes zoster ophthalmicus.