This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
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 - 
Select and explain. 
A. MEN I, B. Pituitary adenoma, C. Adrenal cortical adenoma, D. Ectopic ACTH secreting tumor
D: Ectopic ACTH secreting tumor
Answer: D: Ectopic ACTH secreting tumor 
Explanation: 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.