**Core Concept**
Marfan syndrome is a genetic disorder affecting the body's connective tissue, leading to cardiovascular, skeletal, and ocular complications. The disorder is primarily caused by mutations in the gene encoding fibrillin-1 (FBN1), a crucial protein involved in the formation of elastic fibers found in connective tissue.

**Why the Correct Answer is Right**
The correct mutation associated with Marfan syndrome involves a substitution of glycine (Gly) for another amino acid, typically cysteine (Cys), in the FBN1 gene. This mutation disrupts the formation of elastic fibers, leading to the characteristic features of Marfan syndrome, such as tall stature, joint hypermobility, and aortic root dilatation. The substitution of glycine, an amino acid with a small side chain, for cysteine, an amino acid with a larger side chain, is critical, as glycine is essential for the proper folding of FBN1.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to a mutation in the gene encoding collagen type I (COL1A1), which is associated with osteogenesis imperfecta, not Marfan syndrome.
**Option B:** This option is incorrect because it refers to a mutation in the gene encoding actin (ACTC1), which is associated with hypertrophic cardiomyopathy, not Marfan syndrome.
**Option C:** This option is incorrect because it refers to a mutation in the gene encoding myosin heavy chain (MYH7), which is associated with familial hypertrophic cardiomyopathy, not Marfan syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that Marfan syndrome can be caused by mutations in other genes, including FBN2 and TGFBR1, although these are less common. A thorough family history and genetic testing are crucial for accurate diagnosis and management.

**Correct Answer: C.**