**Core Concept**
Cushing's syndrome is a rare endocrine disorder caused by prolonged exposure to excessive levels of cortisol in the body, leading to various systemic effects. This can be due to an ACTH-producing pituitary tumor (Cushing's disease), an adrenal tumor, or exogenous steroid use.

**Why the Correct Answer is Right**
Cushing's syndrome is characterized by hypercortisolism, which leads to increased gluconeogenesis, lipolysis, and protein catabolism. This results in weight gain, particularly in the central region, hypertension, and glucose intolerance. The excess cortisol also suppresses the hypothalamic-pituitary-adrenal (HPA) axis, reducing ACTH production and leading to secondary adrenal insufficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the pathophysiology of Cushing's syndrome. While it may be a symptom, it is not a defining characteristic of the condition.
**Option B:** This option is incorrect because it is a symptom of Cushing's syndrome rather than a true statement about the condition. The increased cortisol levels lead to various systemic effects, but this option does not accurately capture the underlying mechanism.
**Option C:** This option is incorrect because it is a potential cause of Cushing's syndrome, but it is not a true statement about the condition itself.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Cushing's syndrome can be caused by various factors, including pituitary tumors, adrenal tumors, and exogenous steroid use. It is essential to investigate the underlying cause of hypercortisolism to determine the best course of treatment.

**Correct Answer:** D