**Core Concept**
The body's response to hemorrhage involves the activation of various physiological mechanisms to maintain blood pressure and ensure adequate perfusion of vital organs. This includes the release of vasoactive substances, such as catecholamines, which act on the cardiovascular system to increase heart rate and vascular tone.

**Why the Correct Answer is Right**
In response to hemorrhage, the renin-angiotensin-aldosterone system (RAAS) is activated, leading to increased production of angiotensin II. Angiotensin II is a potent vasoconstrictor that acts on the vascular smooth muscle to increase blood pressure. It also stimulates the release of aldosterone from the adrenal cortex, which promotes sodium retention and water reabsorption in the kidneys, further contributing to increased blood pressure. This increase in blood pressure helps to maintain perfusion of vital organs during hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin is a vasoconstrictor that increases blood pressure, but it is not the primary response to hemorrhage.
**Option B:** Bradykinin is a vasodilator that decreases blood pressure, which is the opposite of what is needed in response to hemorrhage.
**Option C:** Prostacyclin is a vasodilator that inhibits platelet aggregation and increases blood flow, but it does not directly increase blood pressure in response to hemorrhage.

**Clinical Pearl / High-Yield Fact**
The RAAS plays a crucial role in maintaining blood pressure during hemorrhage, and its activation can be seen as a compensatory mechanism to ensure adequate perfusion of vital organs.

**Correct Answer: C. Angiotensin II**