**Core Concept**
Anterior uveitis is a form of inflammation within the uveal tract of the eye, specifically affecting the iris and ciliary body. It is characterized by the presence of inflammatory cells, flare, and increased intraocular pressure.

**Why the Correct Answer is Right**
The earliest sign of anterior uveitis is often the presence of **keratic precipitates (KPs)** on the endothelium of the cornea. KPs are small, white or yellowish deposits of inflammatory cells that accumulate on the corneal endothelium due to the migration of inflammatory cells from the anterior chamber. This occurs as a result of the breakdown of the blood-ocular barrier, allowing inflammatory cells to enter the anterior chamber and accumulate on the corneal endothelium.

**Why Each Wrong Option is Incorrect**
**Option A:** While pain is a common symptom of anterior uveitis, it is not the earliest sign of the condition. Pain typically develops later in the course of the disease as inflammation increases and intraocular pressure rises.
**Option B:** Flare refers to the presence of protein in the aqueous humor, which can be a sign of anterior uveitis. However, flare is typically seen later in the course of the disease and is not the earliest sign.
**Option C:** Hypopyon is the presence of inflammatory cells in the anterior chamber, which can be a sign of anterior uveitis. However, hypopyon is typically seen later in the course of the disease and is not the earliest sign.

**Clinical Pearl / High-Yield Fact**
The "three Ps" of anterior uveitis are photophobia, pain, and perception of light. These symptoms are often present in patients with anterior uveitis and can help guide diagnosis and treatment.

**Correct Answer: A. Keratic precipitates (KPs) on the corneal endothelium.**