**Core Concept**
Scleritis is a severe inflammatory disease of the sclera, the white outer layer of the eye. It is characterized by pain, redness, and swelling of the sclera, and can lead to vision loss if left untreated. The exact cause of scleritis is not well understood, but it is thought to be related to autoimmune disorders, such as rheumatoid arthritis.

**Why the Correct Answer is Right**
Scleritis is a serious condition that requires prompt medical attention. The inflammation associated with scleritis can cause the sclera to thin and weaken, leading to vision loss. The condition is often associated with other autoimmune disorders, such as rheumatoid arthritis, and may be treated with immunosuppressive medications. The sclera is the thickest layer of the eye, and scleritis is a type of anterior uveitis that involves the sclera.

**Why Each Wrong Option is Incorrect**
**Option A:** Scleritis is a type of conjunctivitis. This is incorrect because scleritis involves the sclera, not the conjunctiva, which is a separate layer of tissue covering the white part of the eye and the inside of the eyelids.
**Option B:** Scleritis is a rare disease. This is incorrect because scleritis is a relatively rare condition, but it is not as rare as some other eye diseases.
**Option C:** Scleritis is not associated with autoimmune disorders. This is incorrect because scleritis is often associated with autoimmune disorders, such as rheumatoid arthritis.
**Option D:** Scleritis is a type of posterior uveitis. This is incorrect because scleritis is a type of anterior uveitis that involves the sclera, not the posterior segment of the eye.

**Clinical Pearl / High-Yield Fact**
Scleritis is often associated with other autoimmune disorders, such as rheumatoid arthritis, and may require treatment with immunosuppressive medications. Patients with scleritis may also experience pain, redness, and swelling of the eye, and may have a decreased visual acuity.

**Correct Answer:** C. Scleritis is not associated with autoimmune disorders.