**Core Concept**
Cushing's syndrome is a disorder caused by excess levels of cortisol in the body, leading to various systemic effects. The most common cause of Cushing's syndrome is due to long-term exposure to excess cortisol, which can be exogenous or endogenous in origin.

**Why the Correct Answer is Right**
The most common cause of Cushing's syndrome is exogenous administration of glucocorticoids, such as in the treatment of chronic inflammatory diseases or autoimmune disorders. This is because glucocorticoids are potent hormones that mimic the effects of cortisol. The pituitary gland responds to excess glucocorticoids by reducing ACTH secretion, leading to a decrease in cortisol production. However, when glucocorticoids are administered exogenously, the pituitary gland fails to suppress ACTH production, resulting in excessive cortisol production.

**Why Each Wrong Option is Incorrect**
**Option A:** Endogenous Cushing's syndrome is the second most common cause, accounting for approximately 10-20% of cases. It is characterized by an ACTH-dependent or ACTH-independent cause of cortisol overproduction.
**Option B:** Adrenal tumors are a rare cause of Cushing's syndrome, accounting for less than 5% of cases. They are often non-functioning and require imaging studies to diagnose.
**Option C:** Pituitary tumors are a less common cause of Cushing's syndrome, accounting for approximately 5-10% of cases. They are often ACTH-dependent and require imaging studies to diagnose.

**Clinical Pearl / High-Yield Fact**
The most common cause of Cushing's syndrome is exogenous glucocorticoid administration, and it is essential to consider this in the differential diagnosis of patients with signs and symptoms of Cushing's syndrome.

**Correct Answer: A. Exogenous glucocorticoid administration.**