**Core Concept**
Marfan syndrome is a genetic disorder affecting the body's connective tissue, leading to various systemic manifestations, including ocular, cardiovascular, and skeletal abnormalities. The condition is characterized by the mutation of the FBN1 gene, which codes for fibrillin-1, a protein crucial for the formation of elastic fibers found in connective tissue.

**Why the Correct Answer is Right**
The prominent eye defect in Marfan syndrome is **ectopia lentis**, which refers to the displacement or malposition of the lens within the eye. This occurs due to the weakening of the zonular fibers (suspensory ligaments) that support the lens, leading to its malposition. The FBN1 gene mutation affects the formation of fibrillin-1, which is essential for the proper development and maintenance of the zonular fibers.

**Why Each Wrong Option is Incorrect**
* **Option A:** While glaucoma is a common condition in various eye diseases, it is not a prominent eye defect specifically associated with Marfan syndrome.
* **Option B:** Cataracts can occur in Marfan syndrome, but they are not the most characteristic ocular manifestation of the condition.
* **Option D:** Retinal detachment can be associated with Marfan syndrome, but it is not as specific or prominent as ectopia lentis.

**Clinical Pearl / High-Yield Fact**
Ectopia lentis is often associated with a characteristic "doughnut" or "tear-drop" shape of the pupil, which can be a useful clinical clue for diagnosing Marfan syndrome.

**Correct Answer: C. Ectopia lentis**