**Core Concept**
Giant cell arteritis (GCA) is a large vessel vasculitis that affects the medium and large-sized arteries, including the ophthalmic artery. It is a classic cause of ischemic optic neuropathy, particularly arteritic anterior ischemic optic neuropathy (AION). The ocular manifestations of GCA can be vision-threatening and require prompt diagnosis and treatment.

**Why the Correct Answer is Right**
Arteritic AION is a sudden and painless loss of vision, often accompanied by other systemic symptoms such as headache, scalp tenderness, and jaw claudication. The pathophysiology involves inflammation of the ophthalmic artery and its branches, leading to ischemia of the optic nerve. The presence of giant cells on histopathology is a hallmark of GCA. The American College of Rheumatology (ACR) criteria for diagnosing GCA include the presence of new-onset jaw claudication, scalp tenderness, visual symptoms, or polymyalgia rheumatica, in addition to the presence of temporal artery biopsy showing giant cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Nonarteritic AION is a separate entity that is not associated with GCA, and its pathophysiology involves vasospasm and atherosclerosis rather than inflammation.
**Option C:** Papilledema is swelling of the optic disc due to increased intracranial pressure, which is not a direct manifestation of GCA.
**Option D:** Horner's syndrome is a condition characterized by ptosis, miosis, and anhidrosis due to disruption of the sympathetic fibers, which is not a typical ocular manifestation of GCA.

**Clinical Pearl / High-Yield Fact**
In patients with suspected GCA, a temporal artery biopsy showing giant cells is the gold standard for diagnosis. However, the presence of systemic symptoms such as new-onset jaw claudication, scalp tenderness, or visual symptoms can also support the diagnosis.

**✓ Correct Answer: A. Arteritic AION**