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.
MEN I
Pituitary adenoma
Adrenal cortical adenoma
Ectopic ACTH secreting tumor