**Core Concept**
Turner's syndrome is a genetic disorder in females characterized by the absence of one X chromosome, leading to various cardiovascular and other systemic anomalies. The most common cardiac anomaly in Turner's syndrome is a result of the abnormal development of the aortic arch and the aortic isthmus.

**Why the Correct Answer is Right**
The most common cardiac anomaly in Turner's syndrome is a bicuspid aortic valve. This is due to the abnormal development of the aortic valve leaflets, which are typically tricuspid in normal individuals. The pathophysiology is thought to be related to the abnormal expression of genes involved in valve development, such as the TBX20 and GATA4 genes. The bicuspid aortic valve can lead to aortic stenosis or regurgitation, and is often associated with other cardiovascular anomalies, such as coarctation of the aorta.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while aortic coarctation is a common cardiovascular anomaly in Turner's syndrome, it is not the most common cardiac anomaly. Coarctation of the aorta occurs in approximately 10-15% of individuals with Turner's syndrome.
**Option B:** This option is incorrect because while mitral valve prolapse can occur in Turner's syndrome, it is not the most common cardiac anomaly. Mitral valve prolapse is a condition where the mitral valve leaflets bulge back into the left atrium during systole.
**Option C:** This option is incorrect because while aortic stenosis can occur in Turner's syndrome, it is not the most common cardiac anomaly. Aortic stenosis is a condition where the aortic valve opening is narrowed, leading to obstruction of blood flow from the left ventricle to the aorta.

**Clinical Pearl / High-Yield Fact**
Individuals with Turner's syndrome are at increased risk of aortic dissection and rupture, particularly if they have a bicuspid aortic valve or coarctation of the aorta. This is due to the abnormal development of the aortic wall, which can lead to aortic dilatation and increased risk of dissection.

**Correct Answer: C. Bicuspid aortic valve**