**Core Concept**
Heterochromic uveitis is a rare form of chronic uveitis characterized by inflammation of the uvea, the middle layer of the eye. This condition often presents with heterochromia iridum, where there is a difference in color between the two irises.

**Why the Correct Answer is Right**
The exact pathophysiology of heterochromic uveitis is not well understood, but it is thought to be related to an autoimmune response that affects the uveal tract. The condition often presents with mild symptoms, including blurred vision, eye pain, and sensitivity to light. The heterochromia iridum is due to the destruction of the pigment epithelium of the iris in the affected eye. This condition can be associated with Fuchs heterochromic iridocyclitis, a specific type of heterochromic uveitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because heterochromic uveitis is not typically associated with severe vision loss. Instead, it often presents with mild symptoms and relatively preserved vision.

**Option B:** This option is incorrect because heterochromic uveitis is not primarily caused by a bacterial or fungal infection. While secondary infections can occur, the primary cause is thought to be an autoimmune response.

**Option C:** This option is incorrect because heterochromic uveitis is not typically associated with a sudden onset of symptoms. Instead, it often presents with a gradual onset of mild symptoms.

**Clinical Pearl / High-Yield Fact**
One key feature of heterochromic uveitis is its association with Fuchs heterochromic iridocyclitis, a specific type of heterochromic uveitis characterized by a difference in color between the two irises. This condition often presents with mild symptoms and relatively preserved vision.

**Correct Answer: D.**