**Core Concept:**
The question is testing our knowledge of the pituitary-adrenal axis, the dexamethasone suppression test, and the clinical features of Cushing's syndrome. Cushing's syndrome is a disorder resulting from prolonged exposure to corticosteroids, leading to characteristic clinical features like moon face, buffalo hump, and obesity. The pituitary-adrenal axis involves the hypothalamus releasing corticotropin releasing hormone (CRH), which stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn stimulates the adrenal glands to produce cortisol.

**Why the Correct Answer is Right:**
The correct answer, D. ACTH-dependent Cushing's syndrome, is chosen because the patient does not respond to dexamethasone suppression test. Dexamethasone is a potent synthetic glucocorticoid which suppresses cortisol production when administered in high doses. In a normal individual, a dexamethasone suppression test results in decreased cortisol levels. However, in ACTH-dependent Cushing's syndrome, cortisol levels remain elevated despite dexamethasone administration due to excessive ACTH production.

**Why Each Wrong Option is Incorrect:**
A. Pituitary adenoma (macro-adenoma) is responsible for the elevation of ACTH, not cortisol. This is why the patient doesn't respond to dexamethasone suppression test.
B. Ectopic ACTH syndrome occurs when ACTH is produced outside the pituitary gland. However, this is more likely to cause elevated cortisol levels, not lack of response to dexamethasone suppression test.
C. Adrenal cortical adenoma, also known as Cushing's disease, is caused by an adenoma in the adrenal cortex. This would lead to elevated cortisol levels, not lack of response to dexamethasone suppression test.
D. Primary adrenal insufficiency (Addison's disease) is characterized by low cortisol and aldosterone levels, not elevated cortisol levels and lack of response to dexamethasone suppression test.

**Clinical Pearl:**
The dexamethasone suppression test is a useful tool in diagnosing Cushing's disease, which is an adrenal cause of Cushing's syndrome. By performing this test, we can differentiate between pituitary and adrenal causes of Cushing's syndrome. In pituitary Cushing's disease, the ACTH is suppressed, leading to normal cortisol levels. In adrenal Cushing's disease, cortisol levels remain elevated despite dexamethasone administration.