A young tall, thin male with arachnodactyly has ectopia lentis in both eyes. most likely diagnosis is
**Core Concept**
The question is testing the association between arachnodactyly, a characteristic feature of Marfan syndrome, and ectopia lentis, a common ocular manifestation of this condition. Marfan syndrome is a genetic disorder affecting the body's connective tissue, which provides support and structure to various organs, bones, and other tissues.
**Why the Correct Answer is Right**
Ectopia lentis, or displacement of the lens, occurs due to the weakening of the zonular fibers that suspend the lens in place. In Marfan syndrome, the genetic mutation affecting the fibrillin-1 gene (FBN1) leads to the production of abnormal fibrillin protein, which compromises the integrity of the connective tissue surrounding the lens. This results in the displacement of the lens, causing ectopia lentis. The presence of arachnodactyly, a hallmark feature of Marfan syndrome, further supports this diagnosis.
**Why Each Wrong Option is Incorrect**
* **Option A:** Marfan syndrome primarily affects the cardiovascular system, but it does not typically present with ectopia lentis as a primary feature.
* **Option B:** Ehlers-Danlos syndrome is a group of disorders affecting the body's connective tissue, but it is not typically associated with arachnodactyly or ectopia lentis.
* **Option C:** Homocystinuria is a genetic disorder that can cause ectopia lentis, but it is not typically characterized by arachnodactyly.
**Clinical Pearl / High-Yield Fact**
Remember the "6 Ds" associated with Marfan syndrome: Disproportionate tall stature, Deformities of the skeleton (e.g., arachnodactyly), Dislocation of the lens, Dilation of the aortic root, Dermatospots, and Delayed closure of the epiphyseal plates.
**Correct Answer: C. Homocystinuria**