**Core Concept**
Scleritis is a severe inflammatory condition affecting the sclera, the white outer layer of the eyeball. It is characterized by deep, severe eye pain, redness, and tenderness to touch.

**Why the Correct Answer is Right**
Scleritis often presents with systemic symptoms such as fever, fatigue, and malaise, reflecting its strong association with systemic autoimmune diseases like rheumatoid arthritis, lupus, and scleroderma. The inflammation in scleritis leads to degradation of the scleral collagen, causing thinning and potential perforation of the sclera. The condition can lead to vision loss if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Scleritis is indeed characterized by severe eye pain and redness, making this statement true.
**Option B:** Scleritis is often associated with systemic autoimmune diseases, making this statement true.
**Option C:** Scleritis can lead to vision loss if left untreated, making this statement true.

**Clinical Pearl / High-Yield Fact**
One key distinction between scleritis and episcleritis is the location of the inflammation. Scleritis involves the deeper scleral tissue, whereas episcleritis involves the thinner episcleral tissue.

**Correct Answer:** D.