**Core Concept**
The phenomenon of vasomotor reversal of Dale refers to the observation where a drug initially causes vasoconstriction due to its agonist effects on adrenergic receptors, but subsequent administration of an alpha-adrenergic blocker leads to vasodilation. This is a characteristic of a drug that exhibits both alpha-agonist and alpha-antagonist properties.

**Why the Correct Answer is Right**
The correct answer is a drug that initially stimulates alpha-adrenergic receptors, causing vasoconstriction. However, when an alpha-adrenergic blocker is administered, it binds to and blocks the alpha-adrenergic receptors, leading to a reversal of the vasomotor response and causing vasodilation. This phenomenon is observed in drugs that exhibit both alpha-agonist and alpha-antagonist properties, such as those with intrinsic sympathomimetic activity (ISA).

**Why Each Wrong Option is Incorrect**
* **Option A:** This drug does not exhibit both alpha-agonist and alpha-antagonist properties and therefore does not show the phenomenon of vasomotor reversal of Dale.
* **Option B:** This drug is a pure alpha-agonist and does not have alpha-antagonist properties, so it does not exhibit the phenomenon of vasomotor reversal of Dale.
* **Option D:** This drug is a pure alpha-antagonist and does not have alpha-agonist properties, so it does not exhibit the phenomenon of vasomotor reversal of Dale.

**Clinical Pearl / High-Yield Fact**
When administering alpha-adrenergic blockers, be aware of the potential for vasodilation, especially in patients with pre-existing hypotension or cardiovascular disease. This is because the blockade of alpha-adrenergic receptors can lead to a sudden drop in blood pressure.

**Correct Answer:** B. Prazosin