A 45-year old female had sought medical attention for her recent increase in weight and proximal limb weakness. She was diagnosed with rheumatoid ahritis. 1 week ago has been staed on prednisolone tablets. Physical examamination revealed truncal obesity; BP - 145/90 mm Hg and PR- -90/ mill. High levies of coisol were found in her blood and 24 hour urine sample. A high dose dexamethasone suppression test was performed and the plasma coisol level was suppressed. Serum ACTH level was high. The etiology of this condition can be confirmed by