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What is 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 - and explain in detail?
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.