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?
High-Yield Explanation
Physical examination revealed features of glucocoicoid excess. ACTH-dependent Cushing syndrome is characterized by elevated glucocoicoid levels caused by excess stimulation by ACTH produced by pituitary or ectopic (extrapituitary tissue) tumors. The most frequent source of ectopically produced ACTH is small cell lung carcinoma. Ectopic secretion of ACTH is usually not suppressed by exogenously administered glucocoicoids (dexamethasone), and this feature is helpful in its differential diagnosis.