**Core Concept**
Cushing syndrome is a rare endocrine disorder characterized by excess glucocorticoid levels, leading to various systemic and metabolic effects. The condition arises due to prolonged exposure to cortisol or its synthetic analogs, resulting in a complex interplay of physiological changes.

**Why the Correct Answer is Right**
The most common clinical feature of Cushing syndrome is weight gain, particularly in the central region of the body, often referred to as "moon facies" and "buffalo hump." This is due to the redistribution of body fat, increased appetite, and impaired glucose metabolism. The excess cortisol disrupts the normal feedback mechanisms of the hypothalamic-pituitary-adrenal axis, leading to increased gluconeogenesis, lipolysis, and lipogenesis. As a result, patients experience weight gain, hypertension, and glucose intolerance.

**Why Each Wrong Option is Incorrect**
* **Option A:** While hirsutism is a common feature of Cushing syndrome, it is not the most common clinical presentation. Hirsutism is more characteristic of hyperandrogenic disorders.
* **Option B:** Hypokalemia is a potential complication of Cushing syndrome, but it is not the most common clinical feature. Hypokalemia occurs due to mineralocorticoid excess, which is not a primary characteristic of Cushing syndrome.
* **Option C:** Osteoporosis is a common complication of long-standing Cushing syndrome, but it is not the most common clinical feature. Osteoporosis occurs due to the negative effects of glucocorticoids on bone metabolism.

**Clinical Pearl / High-Yield Fact**
The "Cushingoid" appearance, characterized by weight gain, moon facies, and buffalo hump, is a classic clinical presentation of Cushing syndrome. This distinctive appearance can serve as a valuable diagnostic clue in patients with suspected Cushing syndrome.

**Correct Answer: C. Weight gain**