**Core Concept**
Cushing's syndrome is a rare endocrine disorder caused by excess levels of cortisol in the body, which can result from various factors such as prolonged use of corticosteroid medications, pituitary tumors, adrenal gland tumors, or ectopic ACTH-producing tumors. The excess cortisol leads to a range of clinical manifestations affecting multiple organ systems.

**Why the Correct Answer is Right**
The clinical features of Cushing's syndrome can be broadly categorized into those resulting from cortisol's glucocorticoid effects and those due to mineralocorticoid effects. Glucocorticoid effects include weight gain, particularly in the abdominal area, buffalo hump, purple striae, thinning of the skin, and hypertension. Mineralocorticoid effects lead to hypokalemia and hypertension due to sodium retention. Additionally, Cushing's syndrome can cause glucose intolerance, osteoporosis, and impaired wound healing.

**Why Each Wrong Option is Incorrect**
**Option A:** While weight gain is a common feature of Cushing's syndrome, it is not the only clinical feature. Weight gain, particularly in the abdominal area, is a hallmark of the condition.
**Option B:** Hypokalemia and hypertension are indeed clinical features of Cushing's syndrome, particularly those related to mineralocorticoid effects. Hypokalemia results from excessive sodium retention, leading to potassium loss.
**Option C:** Osteoporosis is a known complication of long-term Cushing's syndrome due to the suppressive effect of cortisol on bone formation.

**Clinical Pearl / High-Yield Fact**
One of the key clinical features of Cushing's syndrome is the presence of purple striae, which are a result of thinning and fragility of the skin due to glucocorticoid excess.

**Correct Answer: D.**