36 year old female , asymptomatic but with raised 24-hour urinary coisol level and raised ACTH with suppression in high dose dexamethasone suppression test. MRI of head reveals no enlargement of the pituitary gland. What should be the next step in this case?

**Core Concept**
The patient is suspected to have Cushing's syndrome due to hypercortisolism, with an abnormal high-dose dexamethasone suppression test suggesting an autonomous cortisol source. The absence of pituitary enlargement on MRI suggests a non-pituitary cause.

**Why the Correct Answer is Right**
The high-dose dexamethasone suppression test is used to differentiate between ACTH-dependent and ACTH-independent causes of Cushing's syndrome. Since the test is suppressed, it points towards an ectopic source of ACTH. The next step involves investigating for an ectopic ACTH-producing tumor, typically starting with imaging of the thorax and abdomen, particularly focusing on small cell lung cancer and neuroendocrine tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the pituitary gland would be unnecessary as the MRI has already ruled out pituitary enlargement.
**Option B:** Measurement of 24-hour urinary free cortisol is already done, and the high-dose dexamethasone suppression test has provided additional diagnostic information.
**Option C:** Insulin tolerance test is not relevant in this case as the diagnosis of Cushing's syndrome is already established.

**Clinical Pearl / High-Yield Fact**
In Cushing's syndrome, the high-dose dexamethasone suppression test is used to differentiate between ACTH-dependent (Cushing's disease and ectopic ACTH-producing tumors) and ACTH-independent (adenocortical tumors) causes.

**Correct Answer: C. Imaging of the thorax and abdomen to investigate for an ectopic ACTH-producing tumor.**

✓ Correct Answer: C. Inferior petrosal sinus venous sampling