**Core Concept**
Cushing's syndrome is a rare endocrine disorder caused by excess levels of cortisol in the body, leading to a variety of systemic effects. It can result from exogenous or endogenous sources, with the latter being more common. The excess cortisol can be due to an overproduction of adrenocorticotropic hormone (ACTH) from a pituitary adenoma or ectopic ACTH-producing tumors.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Cushing's syndrome. In this condition, the excess cortisol has a negative feedback effect on the hypothalamic-pituitary-adrenal (HPA) axis. The elevated cortisol levels suppress the secretion of corticotropin-releasing hormone (CRH) from the hypothalamus and ACTH from the pituitary gland, leading to a decrease in cortisol production. However, in cases of exogenous steroid use, the negative feedback effect is blocked, resulting in continued cortisol production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the pathophysiology of Cushing's syndrome.
**Option B:** This option is incorrect because it is a characteristic of Addison's disease, not Cushing's syndrome.
**Option C:** This option is incorrect because it is a treatment for Cushing's syndrome, not a defining feature.

**Clinical Pearl / High-Yield Fact**
In Cushing's syndrome, weight gain is a common feature, particularly in the trunk and face, due to the redistribution of fat. This is often referred to as "moon facies" and can be a distinguishing feature from other endocrine disorders.

**Correct Answer:** D.