**Core Concept**
Traumatic glaucoma is a condition where increased intraocular pressure (IOP) occurs following blunt or penetrating eye trauma, leading to vision loss and other ocular complications. This is often due to damage to the trabecular meshwork, which is responsible for aqueous humor drainage.

**Why the Correct Answer is Right**
The patient's symptoms of pain in the right eye and loss of vision, following injury to the temple region, are classic signs of traumatic glaucoma. This condition can occur due to the release of aqueous humor from the damaged globe, leading to an increase in IOP. The normal pupillary reaction to light suggests that the afferent pathways are intact, but the damage to the eye's anterior segment has caused the increased IOP.

**Why Each Wrong Option is Incorrect**
**Option A:** Functional loss of vision would not typically present with pain in the eye, and would not be associated with an increase in IOP. Functional loss of vision is a psychogenic cause of visual impairment, and is not related to the physical examination findings in this case.
**Option B:** Optic nerve atrophy would typically present with a history of prolonged increase in IOP, and would likely be associated with abnormalities in the pupillary reaction to light. In this case, the pupil reacts normally to light, making optic nerve atrophy less likely.
**Option D:** Sub-arachnoid hemorrhage is not directly related to the symptoms or examination findings in this case. While it is a potential complication of head trauma, it would typically present with symptoms of increased intracranial pressure, such as headache and altered mental status.

**Clinical Pearl / High-Yield Fact**
Traumatic glaucoma is a medical emergency that requires prompt treatment to prevent further vision loss. The key to diagnosis is the combination of symptoms and signs, including pain in the eye, loss of vision, and normal pupillary reaction to light.

**✓ Correct Answer: C. Traumatic glaucoma**