**Core Concept**
Marfan's syndrome is a genetic disorder caused by mutations in a specific gene, leading to the disruption of the microfibrils in the extracellular matrix of connective tissue. This disruption affects various systems in the body, including the cardiovascular, skeletal, and ocular systems.

**Why the Correct Answer is Right**
The correct answer is C. Fibrillin I. Marfan's syndrome is caused by mutations in the FBN1 gene, which encodes the protein fibrillin-1. Fibrillin-1 is a crucial component of microfibrils, which are essential for the structural integrity of connective tissue. The mutations in the FBN1 gene lead to the production of abnormal fibrillin-1, resulting in the disorganization of microfibrils and the subsequent clinical manifestations of Marfan's syndrome. Fibrillin-1 plays a key role in the formation of elastic fibers, which are essential for maintaining the elasticity of tissues such as blood vessels, skin, and lungs.

**Why Each Wrong Option is Incorrect**
* **Option A:** Collagen I is a major component of the extracellular matrix, but mutations in the COL1A1 or COL1A2 genes are associated with osteogenesis imperfecta, not Marfan's syndrome.
* **Option B:** Collagen IV is a component of basement membranes, and mutations in the COL4A1 or COL4A2 genes are associated with Alport syndrome, not Marfan's syndrome.
* **Option D:** Fibrillin II is a protein that is also involved in the formation of microfibrils, but mutations in the FBN2 gene are associated with congenital contractural arachnodactyly, a rare genetic disorder.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Marfan's syndrome is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the disease. Patients with Marfan's syndrome are at an increased risk of aortic dissection, mitral valve prolapse, and other cardiovascular complications.

**✓ Correct Answer: C. Fibrillin I**