Okay, let's tackle this question about ectopia lentis in a child. The question is asking which disease is associated with ectopia lentis in a pediatric patient. The correct answer is Marfan syndrome, but I need to make sure I explain why that's the case and why the other options aren't.

First, the core concept here is connective tissue disorders. Ectopia lentis is a condition where the lens of the eye is displaced from its normal position. It's a classic sign of certain genetic disorders, especially those affecting connective tissues like Marfan syndrome. I should mention that Marfan syndrome is an autosomal dominant disorder caused by mutations in the FBN1 gene, which codes for fibrillin-1. This protein is crucial for the formation of elastic fibers in connective tissues, and its deficiency leads to structural weaknesses in various parts of the body, including the eye.

Now, why is Marfan syndrome the correct answer? Because one of the hallmark ocular manifestations is ectopia lentis, typically with the lens displaced upward and outward. The absence of strong connective tissue in the zonular fibers that hold the lens in place leads to this dislocation. Other features of Marfan include aortic aneurysms, arachnodactyly, and pectus excavatum. The FBN1 gene mutation is key here, so that's important to note.

For the incorrect options, let's consider other conditions. Homocystinuria is another condition that can cause ectopia lentis, but it's more common in adults and is due to a different metabolic issue—deficiency in cystathionine beta-synthase leading to homocysteine accumulation. It's also associated with mental retardation and thromboembolic events. Osteogenesis imperfecta is a collagen-related disorder causing brittle bones, but the lens issues there are more about opacities rather than dislocation. Ehlers-Danlos syndrome affects connective tissue too but presents with hypermobile joints and skin hyperextensibility, not typically ectopia lentis.

The clinical pearl here is that ectopia lentis in children is a red flag for Marfan syndrome. Remembering the key features like tall stature, arachnodactyly, and cardiovascular issues can help in diagnosis. Also, the Beighton score for joint hypermobility is more associated with Ehlers-Danlos, which differentiates it from Marfan.

I need to make sure the explanation is concise but covers all the necessary points. Check the character count and structure each section properly without markdown headings. Use bold for section titles and keep each part within the specified length. Avoid any markdown except bold and bullets where needed. Let me put it all together now.

**Core Concept**
Ectopia lentis is a displacement of the crystalline lens from its normal position, commonly caused by defects in connective tissue. It is a defining feature of **Marfan syndrome**, an autosomal dominant disorder due to mutations in the **FBN1 gene** (encoding fibrillin-1), which weakens zonular fibers supporting the lens.

**Why the Correct Answer is Right**
In Marfan syndrome, defective fibrillin-1 leads to fragile connective tissue in the eye, causing lens dislocation (typically superior and temporal). Other key features include aortic root dilation, arachnodactyly, and pectus deformities

✓ Correct Answer: B. Homocystinuria