**Core Concept**
The question requires understanding the differential diagnosis of Cushing's syndrome, a condition characterized by excessive cortisol levels, which can be caused by either an ACTH-secreting pituitary tumor or ectopic ACTH production. The key principle here is the ability to distinguish between these two causes based on the response to dexamethasone suppression tests.

**Why the Correct Answer is Right**
The high dose dexamethasone suppression test (HDDST) is the most sensitive test for differentiating between ACTH-secreting pituitary tumors and ectopic ACTH production. This is because ACTH-secreting pituitary tumors typically suppress cortisol production in response to high doses of dexamethasone, whereas ectopic ACTH-producing tumors do not respond to high doses of dexamethasone. The HDDST works by administering a high dose of dexamethasone, which suppresses ACTH secretion in normal individuals and in those with pituitary-dependent Cushing's disease. If cortisol levels fail to decrease after high-dose dexamethasone, it suggests ectopic ACTH production.

**Why Each Wrong Option is Incorrect**
**Option A:** 24 hour Urinary free cortisol is a screening test for Cushing's syndrome, but it does not help differentiate between ACTH-secreting pituitary tumors and ectopic ACTH production.

**Option B:** Overnight dexamethasone suppression test is useful in diagnosing Cushing's syndrome, but it is not sensitive enough to differentiate between the two causes mentioned in the question.

**Option C:** Low dose dexamethasone suppression test is also useful in diagnosing Cushing's syndrome, but it is not as sensitive as the high dose test in differentiating between ACTH-secreting pituitary tumors and ectopic ACTH production.

**Clinical Pearl / High-Yield Fact**
The high dose dexamethasone suppression test is a critical tool in the differential diagnosis of Cushing's syndrome, and failure to suppress cortisol levels after high-dose dexamethasone is highly suggestive of ectopic ACTH-producing tumors.

**✓ Correct Answer: D. High dose dexamethasone suppression test**