**Core Concept**
Anterior uveitis is characterized by inflammation of the uveal tract's anterior segment, primarily affecting the iris and ciliary body. However, posterior segment involvement can lead to complications such as cystoid macular edema (CME), vitreous inflammation, and retinal vasculitis, resulting in decreased vision.

**Why the Correct Answer is Right**
Posterior segment involvement in anterior uveitis can lead to CME, a condition where fluid accumulates in the macula due to increased vascular permeability and impaired lymphatic drainage. This results in retinal thickening, vision distortion, and decreased visual acuity. The pathogenesis involves the release of pro-inflammatory cytokines, such as TNF-alpha and IL-1 beta, which increase vascular permeability and lead to fluid accumulation in the macula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not directly relate to the posterior segment involvement in anterior uveitis. Cataract formation is a possible complication of chronic uveitis, but it is not the primary cause of decreased vision in this context.
**Option B:** Vitreous hemorrhage is a possible complication of posterior segment involvement in uveitis, but it is not the most common cause of decreased vision in patients with anterior uveitis.
**Option C:** Retinal detachment is a serious complication of posterior segment involvement in uveitis, but it is not the most direct cause of decreased vision in patients with anterior uveitis.

**Clinical Pearl / High-Yield Fact**
Cystoid macular edema (CME) is a common complication of posterior segment involvement in anterior uveitis, and its early detection and treatment are crucial to prevent vision loss.

**Correct Answer: C. Retinal vasculitis.**