**Core Concept**
Marfan's syndrome is a genetic disorder caused by a mutation in the fibrillin-1 (FBN1) gene, which codes for a glycoprotein essential for the formation of elastic fibers found in connective tissue. This mutation affects the structure and function of elastic fibers, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The correct answer is a mutation in the FBN1 gene, which encodes the fibrillin-1 protein. This protein plays a crucial role in the formation of elastic fibers in connective tissue, such as in the aorta, skin, and eyes. The mutation disrupts the normal structure and function of elastic fibers, leading to the characteristic features of Marfan's syndrome, including tall stature, arachnodactyly, and aortic root dilatation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct gene associated with Marfan's syndrome.
**Option B:** This option is incorrect because it mentions a different gene, not related to Marfan's syndrome.
**Option C:** This option is incorrect because it does not accurately describe the genetic mutation responsible for Marfan's syndrome.

**Clinical Pearl / High-Yield Fact**
A key feature of Marfan's syndrome is the association between the genetic mutation and aortic root dilatation, which can lead to aortic dissection and rupture. This emphasizes the importance of regular echocardiographic monitoring in patients with Marfan's syndrome.

**Correct Answer:** A. FBN1.