**Core Concept**
Anterior uveitis is a form of eye inflammation that primarily affects the anterior segment of the eye, which includes the iris, ciliary body, and the anterior chamber. The earliest lesion in anterior uveitis is associated with the breakdown of the blood-aqueous barrier, leading to the accumulation of inflammatory cells and proteins in the aqueous humor.

**Why the Correct Answer is Right**
The earliest lesion in anterior uveitis is characterized by the disruption of the blood-aqueous barrier, which is mediated by the breakdown of tight junctions between the endothelial cells lining the ciliary body. This disruption allows the passage of plasma proteins and inflammatory cells from the bloodstream into the aqueous humor. The increased permeability of the blood-aqueous barrier is a result of the release of pro-inflammatory cytokines and mediators, such as IL-1β and TNF-α, which induce the expression of adhesion molecules and increase the permeability of the endothelial cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the breakdown of the blood-aqueous barrier is not the initial step in the pathogenesis of anterior uveitis.
* **Option B:** This option is incorrect because the accumulation of inflammatory cells in the anterior chamber is a secondary event that occurs as a result of the breakdown of the blood-aqueous barrier.
* **Option C:** This option is incorrect because the release of inflammatory mediators is a downstream event that occurs as a result of the disruption of the blood-aqueous barrier.

**Clinical Pearl / High-Yield Fact**
The breakdown of the blood-aqueous barrier is a key event in the pathogenesis of anterior uveitis, and it is often accompanied by the release of inflammatory mediators, such as IL-1β and TNF-α, which contribute to the recruitment of inflammatory cells to the anterior chamber.

**Correct Answer: B. Accumulation of inflammatory cells in the anterior chamber.**