**Core Concept**
The patient's presentation suggests Cushing's syndrome, characterized by excess cortisol production. Dexamethasone suppression testing is a diagnostic tool to differentiate between ACTH-dependent and ACTH-independent causes of Cushing's syndrome.

**Why the Correct Answer is Right**
The failure of dexamethasone to suppress cortisol secretion indicates an ACTH-dependent cause of Cushing's syndrome. This pattern is typically seen in Cushing's disease, where a pituitary adenoma secretes excess ACTH. The next step in management involves localizing the source of ACTH production, which is usually a pituitary adenoma. High-resolution pituitary imaging, such as MRI, is the most appropriate next step to identify the adenoma.

**Why Each Wrong Option is Incorrect**
* **Option A:** CT scan of the adrenal glands is not indicated as the primary step, as the elevated cortisol levels are likely due to ACTH stimulation, not adrenal gland pathology.
* **Option B:** Measurement of plasma ACTH levels would be a useful follow-up test but is not the next step in management after failure of dexamethasone suppression testing.
* **Option D:** Further medical management with higher doses of dexamethasone or other medications is not the next step, as the primary goal is to localize and treat the underlying cause of Cushing's syndrome.

**Clinical Pearl / High-Yield Fact**
Remember the "Atkinson's Criteria" for differentiating between Cushing's disease and ectopic ACTH-producing tumors: failure of high-dose dexamethasone suppression is more common in Cushing's disease.

**Correct Answer:** C. MRI of the pituitary gland.