**Core Concept:** Turner's syndrome is a genetic disorder characterized by partial or complete absence of one X chromosome in females. It is associated with various cardiac, renal, and skeletal anomalies.

**Why the Correct Answer is Right:** In Turner's syndrome, the affected individual has a deficiency of functional estrogen due to the impaired function of ovaries. This results in a decrease in the development of the coronary arteries. Consequently, the heart is subjected to increased blood flow, leading to the formation of arterial ectasia - dilation of blood vessels due to increased blood flow. The most common cardiac anomaly in Turner's syndrome is thus arterial ectasia, which can cause congestive heart failure, pulmonary hypertension, and arrhythmias.

**Why Each Wrong Option is Incorrect:**

A. **Defective valves**: While some patients with Turner's syndrome may have coarctation of the aorta or stenosis of the pulmonary artery, the correct answer explains the overall cardiovascular anomaly, not just valvular defects.

B. **Hypertrophic cardiomyopathy**: Although this condition can be seen in Turner's syndrome, it is less common than arterial ectasia and does not explain the overall cardiovascular anomaly.

C. **Congenital heart defects**: While congenital heart defects may be present in Turner's syndrome, arterial ectasia is the most common cardiac anomaly and best explains the overall cardiovascular anomaly.

D. **Heart failure**: This is a consequence of the cardiac anomalies but not the primary anomaly in Turner's syndrome.

**Clinical Pearl:** Turner's syndrome is a reminder that genetics plays a significant role in understanding various congenital heart defects and their mechanisms. A thorough understanding of the underlying pathophysiology is crucial for accurate diagnosis and management.