**Core Concept**
Ectopia lentis is a congenital or acquired displacement of the lens from its normal anterior-posterior position in the eye. This condition can be caused by various factors, including genetic disorders, trauma, and systemic diseases.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of ectopia lentis. In Marfan syndrome, a genetic disorder affecting the connective tissue, the zonular fibers that suspend the lens in the eye are weakened, leading to lens dislocation. This is due to the mutation in the FBN1 gene, which codes for fibrillin-1, a protein crucial for the formation of elastic fibers in connective tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ectopia lentis is not typically associated with diabetes mellitus. While diabetes can cause cataracts and other eye problems, it is not a primary cause of lens dislocation.

**Option B:** This option is incorrect because glaucoma is a group of eye conditions that damage the optic nerve, but it is not a direct cause of ectopia lentis.

**Option C:** This option is incorrect because rheumatoid arthritis is a systemic autoimmune disease that can cause various eye problems, including dry eye and scleritis, but it is not a primary cause of ectopia lentis.

**Clinical Pearl / High-Yield Fact**
Ectopia lentis can be associated with other systemic conditions, such as homocystinuria, Weill-Marchesani syndrome, and Ehlers-Danlos syndrome. These conditions often present with a combination of ocular and systemic features, making a thorough medical and ocular examination essential for diagnosis.

**Correct Answer: C. Marfan syndrome**