A chronic smoker presents with mild haemoptysis. He also gives a history of hypeension and obesity. Lab data showed raised ACTH levels, which were not suppressed by dexamethasone. A diagnosis of Cushing's syndrome is made. Which among the following is the cause of Cushing's syndrome in the patient ?

The main clues in this question are that the patient is a smoker with hemoptysis, has high ACTH level which is not suppressed by dexamethasone. Cushings syndrome could be due to ACTH hypersecretion by the pituitary in 40%, non pituitary ACTH producing neoplasm in 10% and in 30% due to excessive autonomous secretion of coisol by the adrenals independent of ACTH. Since this patient has high ACTH levels the underlying cause could be either pituitary adenoma or non pituitary neoplasm producing ACTH like small cell lung cancer. As this patient is a symptomatic smoker the most likely cause would be a small cell lung cancer. And this ACTH cannot be suppressed by dexamethasone. ACTH produced by pituitary neoplasm can be suppressed by dexamethasone. Ref: CURRENT Medical Diagnosis & Treatment 2014 chapter 26.