**Core Concept**
Cardiovascular manifestations are a common feature of various systemic diseases, particularly those affecting the endocrine system. The autonomic nervous system plays a crucial role in regulating cardiovascular function, and imbalances in this system can lead to cardiovascular complications. In the context of the given options, we need to identify which systemic disease is most likely to exhibit cardiovascular manifestations as a dominant feature.

**Why the Correct Answer is Right**
The correct answer is associated with pheochromocytoma, a rare tumor of the adrenal gland that secretes excess catecholamines, such as epinephrine and norepinephrine. These hormones stimulate the heart, leading to increased heart rate, blood pressure, and cardiac output. The resulting cardiovascular manifestations include hypertension, tachycardia, cardiac arrhythmias, and even cardiac failure. The excessive sympathetic stimulation can also cause vasoconstriction, further contributing to the development of hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, while thyroid disorders can affect the cardiovascular system, they are not typically characterized by dominant cardiovascular manifestations. Thyroid hormone imbalances can lead to changes in cardiac function and blood pressure, but these effects are usually less pronounced than those seen in pheochromocytoma.

**Option B:** This option is incorrect because, although diabetes mellitus can lead to cardiovascular complications, such as atherosclerosis and cardiac failure, these manifestations are not as acute or dramatic as those seen in pheochromocytoma. The cardiovascular effects of diabetes are generally more insidious and develop over a longer period.

**Option C:** This option is incorrect because, while hyperparathyroidism can lead to hypertension and cardiac calcification, these effects are not as directly related to the autonomic nervous system as those seen in pheochromocytoma. The cardiovascular manifestations of hyperparathyroidism are generally less prominent.

**Option D:** This option is incorrect because, although Cushing's syndrome can lead to hypertension and cardiac effects, these are not as dominant as those seen in pheochromocytoma. The cardiovascular manifestations of Cushing's syndrome are generally more complex and multifactorial.

**Clinical Pearl / High-Yield Fact**
Pheochromocytomas are often referred to as the "10% tumor" because they account for only about 10% of all adrenal gland tumors, but they are responsible for a significant proportion of cases of secondary hypertension.

**Correct Answer:** C. Pheochromocytoma.