**Core Concept**
The question requires identifying a pharmacological agent that primarily causes vasodilation in the venous system, rather than the arterial system. This is a key concept in pharmacology, as venodilators can increase cardiac output and reduce venous pressure.

**Why the Correct Answer is Right**
The correct answer is likely a medication that acts on the venous system, causing smooth muscle relaxation and vasodilation. This can be achieved through various mechanisms, such as direct smooth muscle relaxation or by inhibiting the reuptake of certain neurotransmitters. For example, the medication nitroglycerin is a classic venodilator, acting by releasing nitric oxide, which relaxes smooth muscle in the venous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a medication that primarily causes arterial vasodilation, such as hydralazine. While it may have some effects on the venous system, its primary action is on the arterial system.
**Option B:** This option is incorrect because it is a medication that primarily causes vasoconstriction, such as ergotamine. It is not a venodilator.
**Option C:** This option is incorrect because it is a medication that primarily affects the heart rate and contractility, such as dobutamine. While it may have some effects on the venous system, its primary action is on the heart.

**Clinical Pearl / High-Yield Fact**
Nitroglycerin is a classic example of a venodilator, and its primary use is to reduce angina symptoms by increasing cardiac output and reducing venous pressure. This is why it is often used in the treatment of acute coronary syndrome.

**Correct Answer:** C.