**Core Concept**
Fuchs' heterochromic iridocyclitis is a rare, idiopathic form of uveitis characterized by chronic inflammation of the iris and ciliary body. The condition is marked by a heterochromia, or difference in color, between the two eyes.

**Why the Correct Answer is Right**
The hallmark feature of Fuchs' heterochromic iridocyclitis is the presence of heterochromia, particularly in the iris. This is due to chronic inflammation and atrophy of the iris sphincter and dilator muscles, leading to a change in the color of the iris. Additionally, patients with Fuchs' heterochromic iridocyclitis often have a mild, non-granulomatous uveitis, which is characterized by a lack of significant inflammation and a relatively good prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Fuchs' heterochromic iridocyclitis is not typically associated with significant pain or vision loss, unlike other forms of uveitis.

**Option B:** This option is incorrect because Fuchs' heterochromic iridocyclitis is not a form of granulomatous uveitis, which is characterized by a more severe inflammation and the presence of granulomas.

**Option C:** This option is incorrect because Fuchs' heterochromic iridocyclitis is not typically associated with a significant increase in intraocular pressure (IOP), unlike other forms of uveitis.

**Clinical Pearl / High-Yield Fact**
Fuchs' heterochromic iridocyclitis is often described as a "lazy eye" due to its chronic and mild nature, but it can still cause significant heterochromia and vision loss if left untreated.

**Correct Answer:** C.