Okay, let's tackle this question. The patient has central obesity, buffalo hump, and purple striae—classic signs of Cushing's syndrome. But the key is the lab results. Both ACTH and cortisol are elevated. The high-dose dexamethasone test shows 75% suppression of cortisol. Hmm.

Cushing's can be caused by different things: pituitary (Cushing's disease), adrenal tumors, or ectopic ACTH. The ACTH levels are important here. If ACTH is high, it's likely a pituitary issue because it would stimulate the adrenal glands. Adrenal tumors usually produce cortisol independently and suppress ACTH. So high ACTH and cortisol with suppression on high-dose dexamethasone points to a pituitary source. Wait, high-dose dexamethasone suppression is typical for Cushing's disease. If it was an adrenal tumor, the suppression wouldn't happen because the ACTH isn't the driver there. The options probably include Cushing's disease, adrenal adenoma, ectopic ACTH, and maybe adrenal hyperplasia. The correct answer here is Cushing's disease. The other options don't fit the lab results. Adrenal tumors don't have elevated ACTH. Ectopic ACTH would have high ACTH and cortisol but not suppress with dexamethasone. So the high suppression with high-dose dexamethasone is key here.

**Core Concept**
This question assesses the differential diagnosis of Cushing’s syndrome, focusing on distinguishing between ACTH-dependent (pituitary or ectopic) and ACTH-independent (adrenal) causes. The dexamethasone suppression test is critical for localization.

**Why the Correct Answer is Right**
The patient’s clinical features (central obesity, buffalo hump, striae) and elevated ACTH/cortisol suggest ACTH-dependent Cushing’s. The **75% suppression of cortisol** with high-dose dexamethasone indicates **Cushing’s disease** (pituitary adenoma). This occurs because high-dose dexamethasone feedback-inhibits the pituitary, reducing ACTH secretion. In contrast, adrenal tumors or ectopic ACTH sources are not suppressed by dexamethasone.

**Why Each Wrong Option is Incorrect**
**Option A:** Adrenal adenoma (ACTH-independent) would present with **low ACTH** due to negative feedback, not elevated ACTH.
**Option B:** Ectopic ACTH syndrome (e.g., small cell carcinoma) causes **no suppression** on dexamethasone testing.
**Option C:** Adrenal hyperplasia (ACTH-dependent) is rare and typically shows **partial suppression** with low-dose dexamethasone, not high-dose.

**Clinical Pearl / High-Yield Fact**
**High-dose dexamethasone suppression test** differentiates pituitary (Cushing’s disease) from ectopic ACTH. Remember: **"High suppresses the pituitary, low suppresses the adrenal."** Always correlate suppression results with ACTH levels.

**Correct Answer: C. Cushing’s disease**

✓ Correct Answer: D. Cushing's disease