**Core Concept**
The patient's presentation of weight gain, purple stretch marks, thinning of the skin, bruising, and proximal muscle weakness is suggestive of Cushing's syndrome, a condition characterized by excess cortisol levels. This is due to the overproduction of cortisol, which can lead to a range of systemic effects, including changes in body composition and skin integrity.

**Why the Correct Answer is Right**
The patient's reduced serum cortisol and plasma ACTH levels are indicative of Cushing's disease, which is caused by an ACTH-producing pituitary adenoma leading to bilateral adrenal hyperplasia and cortisol overproduction. However, in this case, the reduced ACTH levels suggest that the cortisol overproduction is due to a different cause, likely an adrenal tumor producing cortisol autonomously (Cushing's syndrome due to adrenal cause). The electrolyte workup will most likely reveal hypokalemia, a common feature of Cushing's syndrome due to cortisol's mineralocorticoid effects on the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not explain the patient's reduced ACTH levels, which are not typical of primary adrenal causes of Cushing's syndrome.
**Option B:** This option is incorrect because it does not account for the patient's reduced ACTH levels, which are not characteristic of ectopic ACTH-producing tumors.
**Option C:** This option is incorrect because it does not explain the patient's reduced serum cortisol levels, which are not typical of pituitary causes of Cushing's disease.

**Clinical Pearl / High-Yield Fact**
Cushing's syndrome due to adrenal causes can be differentiated from pituitary causes by the presence of reduced ACTH levels and the absence of pituitary enlargement on imaging studies.

**Correct Answer:** C.