## **Core Concept**
The patient's presentation of fatigue, weakness, weight gain, central obesity, skin striae, and hypertension suggests Cushing's syndrome, a condition caused by excess cortisol. The diagnosis involves differentiating between the causes of Cushing's syndrome, such as ACTH-dependent and ACTH-independent forms.

## **Why the Correct Answer is Right**
The key to this question lies in interpreting the laboratory results provided. The patient has low ACTH levels (<50 pg/mL), high plasma cortisol (35 μg/dL), and high urine 17-OH and 17-KS levels. The lack of response to ACTH IV, dexamethasone 0.5 mg and 2.0 mg, and metyrapone indicates an autonomous source of cortisol production not responsive to normal feedback mechanisms. This profile is consistent with **Cushing's syndrome due to adrenal cause (Adrenal Cushing's)**, likely an adrenal adenoma or carcinoma, which is ACTH-independent. ## **Why Each Wrong Option is Incorrect** - **Option A (Cushing's disease):** This is ACTH-dependent Cushing's syndrome, usually caused by a pituitary adenoma. The patient's low ACTH level rules out this option. - **Option B (Ectopic ACTH-producing tumor):** This is another form of ACTH-dependent Cushing's syndrome. The low ACTH level and the patient's failure to respond to dexamethasone suppression test make this less likely. - **Option D (Familial Cushing's syndrome):** This would typically present with a different set of clinical and biochemical findings and often at a younger age. The acute presentation and lab findings do not support a familial or genetic cause. ## **Clinical Pearl / High-Yield Fact** A critical point to remember is that in **Adrenal Cushing's syndrome**, you expect to see **low ACTH levels** because the adrenal gland is autonomously producing cortisol, which then feedback inhibits ACTH production. This distinguishes it from ACTH-dependent causes, where ACTH levels would be inappropriately normal or elevated. ## **Correct Answer:** C.