**Core Concept**
Cushing's syndrome is a disorder caused by excess cortisol production, leading to various systemic effects. The commonest cause of Cushing's syndrome is an overproduction of adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to produce cortisol.

**Why the Correct Answer is Right**
ACTH, produced by the pituitary gland, stimulates the adrenal glands to produce cortisol. In Cushing's syndrome, an excess of ACTH leads to an overproduction of cortisol. This is often due to a pituitary adenoma, also known as Cushing's disease, which is a benign tumor of the pituitary gland. Other causes of ACTH-dependent Cushing's syndrome include ectopic ACTH-producing tumors, such as small cell lung cancer. The correct answer highlights the importance of ACTH in the pathophysiology of Cushing's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Low-dose dexamethasone suppression is used to diagnose Cushing's syndrome, but it is not a characteristic of the condition. In fact, patients with Cushing's syndrome typically have an impaired response to dexamethasone suppression.
**Option B:** Adrenal carcinoma is a rare cause of Cushing's syndrome, accounting for less than 10% of cases. Adrenal adenoma is a more common cause, but still less common than ACTH-dependent causes.
**Option C:** Pituitary adenoma size is not a determining factor in the diagnosis of Cushing's disease. Small pituitary adenomas can cause significant ACTH overproduction, while large adenomas may not necessarily be associated with Cushing's disease.

**Clinical Pearl / High-Yield Fact**
When diagnosing Cushing's syndrome, it is essential to measure 24-hour urinary free cortisol or midnight cortisol levels to confirm the diagnosis. Additionally, a high-dose dexamethasone suppression test can help differentiate between ACTH-dependent and ACTH-independent causes of Cushing's syndrome.

**✓ Correct Answer: D. ACTH secretion is the commonest cause**