**Core Concept**
Mooren's ulcer is a rare, idiopathic inflammatory condition affecting the cornea, often resulting in vision loss. This condition is characterized by a painful, progressive corneal ulceration that can lead to significant morbidity if left untreated.

**Why the Correct Answer is Right**
Mooren's ulcer primarily affects the cornea, which is the transparent outer layer of the eye responsible for refracting light. The condition is characterized by a necrotizing keratitis, with the corneal epithelium and stroma being the primary sites of inflammation. This leads to corneal thinning, perforation, and ultimately, vision loss. The involvement of the cornea is a hallmark of Mooren's ulcer, distinguishing it from other ocular inflammatory conditions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mooren's ulcer is typically a painful condition, not painless. The pain is often severe and can be a significant symptom guiding diagnosis and treatment.
* **Option C:** While sudden loss of vision can occur in Mooren's ulcer due to corneal perforation or severe inflammation, it is not a defining characteristic of the condition. Vision loss is often gradual and may precede the onset of sudden vision loss.
* **Option D:** Mooren's ulcer is typically unilateral, affecting one eye at a time. Bilateral involvement is not the majority scenario, although it can occur in some cases.

**Clinical Pearl / High-Yield Fact**
Mooren's ulcer is a classic example of an idiopathic inflammatory condition that can lead to significant vision loss if left untreated. Early recognition and treatment are crucial to prevent corneal perforation and vision loss.

**✓ Correct Answer: B. Affects cornea**