**Core Concept**
Nitric oxide (NO), prostacyclin, and adenosine are key vasodilators involved in the regulation of vascular tone. They exert their effects through various mechanisms, including the relaxation of vascular smooth muscle cells and the inhibition of platelet aggregation.

**Why the Correct Answer is Right**
The correct answer is a substance that does not primarily cause vasodilation. Vasopressin, also known as antidiuretic hormone (ADH), is a vasoconstrictor that causes blood vessels to constrict, leading to increased blood pressure. It acts through V1 receptors on vascular smooth muscle cells, resulting in increased intracellular calcium and contraction of the muscle cells. In contrast, vasodilators like nitric oxide, prostacyclin, and adenosine work to relax vascular smooth muscle cells and decrease blood pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Nitric oxide (NO) is a potent vasodilator that acts by stimulating guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP), leading to relaxation of vascular smooth muscle cells.
* **Option B:** Prostacyclin is a prostaglandin that causes vasodilation by activating adenylate cyclase to produce cyclic adenosine monophosphate (cAMP), leading to relaxation of vascular smooth muscle cells.
* **Option D:** Adenosine is a vasodilator that acts by stimulating adenylate cyclase to produce cAMP, leading to relaxation of vascular smooth muscle cells.

**Clinical Pearl / High-Yield Fact**
In clinical practice, vasodilators like nitroglycerin are often used to treat angina pectoris by reducing myocardial oxygen demand and increasing blood flow to the heart muscle.

**Correct Answer:** D. Vasopressin.