**Core Concept**
The patient's presentation with Cushing's syndrome, characterized by raised ACTH levels not suppressed by dexamethasone, suggests an ectopic source of ACTH production. This is in contrast to endogenous Cushing's syndrome, which typically involves a pituitary or adrenal cause and is responsive to high-dose dexamethasone suppression.

**Why the Correct Answer is Right**
The patient's history of mild haemoptysis, a chronic smoker, and raised ACTH levels not suppressed by dexamethasone point towards an ectopic ACTH-producing tumor. This is often seen in small-cell lung carcinoma (SCLC), which is a type of neuroendocrine tumor that can secrete various hormones, including ACTH. The tumor's ectopic production of ACTH leads to Cushing's syndrome, which is characterized by hypercortisolism and its associated clinical features.

**Why Each Wrong Option is Incorrect**
**Option A:** Multiple endocrine neoplasia type 1 (MEN I) is a hereditary disorder that typically involves tumors of the parathyroid glands, pancreas, and pituitary gland. While MEN I can cause Cushing's syndrome due to a pituitary adenoma, the patient's presentation does not suggest a familial syndrome.

**Option B:** Pituitary adenoma is a common cause of endogenous Cushing's syndrome, which would typically be responsive to high-dose dexamethasone suppression. However, the patient's raised ACTH levels not suppressed by dexamethasone suggest an ectopic source.

**Option C:** Adrenal cortical adenoma can cause Cushing's syndrome due to autonomous cortisol production, but this would not explain the raised ACTH levels.

**Clinical Pearl / High-Yield Fact**
In patients with Cushing's syndrome, a history of smoking and haemoptysis should raise suspicion for an ectopic ACTH-producing tumor, particularly small-cell lung carcinoma. Remember to consider this diagnosis in patients with unexplained Cushing's syndrome and a smoking history.

**✓ Correct Answer: D. Ectopic ACTH secreting tumor**