## **Core Concept**
The patient's symptoms, including weight gain, purple stretch marks (striae), thinning of the skin, bruising, proximal muscle weakness, and hypertension, are suggestive of Cushing's syndrome. Cushing's syndrome is a condition caused by excess levels of cortisol in the body. The laboratory findings of low serum cortisol and low plasma ACTH (Adrenocorticotropic hormone) levels point towards adrenal Cushing's syndrome, likely due to an adrenal tumor.

## **Why the Correct Answer is Right**
The patient's presentation and lab findings are consistent with adrenal Cushing's syndrome. In adrenal Cushing's syndrome, the adrenal glands produce excess cortisol, leading to negative feedback on the pituitary gland, which then decreases production of ACTH. The electrolyte workup in a patient with adrenal Cushing's syndrome is likely to reveal hypokalemia (low potassium levels) due to the mineralocorticoid effects of cortisol (or possibly the tumor producing other steroids). Cortisol can cause potassium excretion in the urine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not specify an electrolyte abnormality but given the context, it's less likely to directly relate to the typical electrolyte disturbances seen in Cushing's syndrome.
- **Option B:** Hypercalcemia (elevated calcium levels) is not typically associated with Cushing's syndrome directly. While there could be secondary effects on calcium metabolism, it's not the most directly related electrolyte abnormality.
- **Option C:** This option is likely correct given the context (see **Why the Correct Answer is Right**), but let's evaluate the rest.
- **Option D:** Hyperphosphatemia (elevated phosphate levels) could occur due to various reasons but is not the most characteristic electrolyte disturbance in Cushing's syndrome.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl in Cushing's syndrome is the presence of **hypokalemic metabolic alkalosis** alongside hypertension, which is a clue towards the diagnosis, especially when considering an electrolyte workup. The mineralocorticoid effects of excess cortisol lead to potassium wasting and sodium retention.

## **Correct Answer:** C. Hypokalemia.