**Core Concept**
Marfan's Syndrome is a genetic disorder affecting the body's connective tissue, leading to various cardiovascular and musculoskeletal complications. The condition is caused by mutations in the FBN1 gene, which codes for fibrillin-1, a protein crucial for the formation of elastic fibers in connective tissue.

**Why the Correct Answer is Right**
The aortic root dilation is a hallmark of Marfan's Syndrome, often presenting as the most common cardiovascular abnormality. This occurs due to the weakening of the aortic wall's elastic fibers, which are essential for maintaining the aorta's structural integrity. The subsequent dilation increases the risk of aortic dissection and rupture. The pathophysiology involves the defective fibrillin-1 protein, which disrupts the normal assembly of elastic fibers, leading to aortic wall weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitral valve prolapse is a common finding in Marfan's Syndrome, but it is not the most common cardiovascular abnormality on presentation.
**Option B:** Atrial septal defect (ASD) can be associated with Marfan's Syndrome, but it is less common than aortic root dilation.
**Option C:** Pulmonary artery dilation is not a characteristic feature of Marfan's Syndrome, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that Marfan's Syndrome patients often have a tall, slender build, with long limbs and fingers. This physical characteristic, combined with a positive family history, can raise suspicion for the condition.

**Correct Answer:** A. Aortic root dilation.