In a chronic smoker with mild haemoptysis. He also gave a history of hypertension and obesity. Lab data showed raised ACTH levels, which were not suppressed by dexamethasone. The cause for the Cushing's syndrome in the patient is -

Remember this In normal person   →  Low dose dexamethasone suppresses cortisol secretion.                                                                               In Cushing's syndromes  → Low dose dexamethasone is unable to suppress cortisol secretion. When Cushing's syndrome is caused due to ectopic ACTH production  →  Even high dose dexamethasone is unable to suppress cortisol secretion.