**Core Concept**
Ectopia lentis refers to the displacement or malposition of the crystalline lens in the eye. This condition can result from various causes, including trauma, systemic diseases, or congenital anomalies. The lens is normally suspended by zonular fibers and is surrounded by the capsular bag.

**Why the Correct Answer is Right**
In Marfan syndrome, a genetic disorder affecting the body's connective tissue, the zonular fibers can be weakened, leading to lens subluxation or dislocation. This occurs due to the defective fibrillin-1 protein, which is essential for the formation of elastic fibers in the zonular apparatus. As a result, the lens loses its normal suspension, and ectopia lentis develops. Other conditions like Weil's disease, congenital rubella, or trauma can also cause ectopia lentis, but Marfan syndrome is a well-known association.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while diabetes can cause cataracts, it is not a primary cause of ectopia lentis. Diabetes can lead to cataract formation due to the accumulation of sorbitol in the lens.
**Option B:** This option is incorrect because glaucoma is a group of eye conditions that damage the optic nerve, often due to increased intraocular pressure. While glaucoma can lead to vision loss, it is not a cause of ectopia lentis.
**Option C:** This option is incorrect because retinal detachment occurs when the retina separates from the underlying tissue. While retinal detachment can cause vision loss, it is not directly related to ectopia lentis.

**Clinical Pearl / High-Yield Fact**
When examining a patient with suspected ectopia lentis, it's essential to perform a thorough systemic evaluation, including checking for signs of Marfan syndrome, such as tall stature, arachnodactyly, and aortic root dilatation.

**Correct Answer: C. Marfan syndrome.**