A 52-year-old construction worker had an 8-month history of muscle weakness, easy bruising, backache, and headache. Physical examination revealed cutaneous hyperpigmentation, pronounced truncal obesity, purple striae with a "buffalo hump," and blood pressure of 180/100 mm Hg. Laboratory analyses revealed elevated concentrations of circulating coisol with an absence of a circadian rhythm. With high-dose administration of a glucocoicoid agonist, plasma coisol levels reduced significantly. What is the most likely cause of these symptoms?