**Core Concept**
Inotropic agents are drugs that modify the force of cardiac contractions, which is essential for maintaining cardiac output in various clinical scenarios. These agents can either increase or decrease the contractility of the heart muscle, depending on their mechanism of action.

**Why the Correct Answer is Right**
The correct answer is a vasodilator, which primarily decreases peripheral vascular resistance rather than directly increasing cardiac contractility. However, the question asks for an inotropic agent, which implies a direct effect on the heart muscle's contractility. The other options are inotropic agents, but the correct answer is not.

**Why Each Wrong Option is Incorrect**
**Option A:** Dobutamine is a beta-1 adrenergic agonist that increases heart rate, contractility, and cardiac output by stimulating the Gs protein-coupled receptor, leading to an increase in cAMP and subsequent contraction of the heart muscle.
**Option B:** Digoxin is a cardiac glycoside that inhibits the Na+/K+-ATPase pump, leading to an increase in intracellular calcium and thereby increasing cardiac contractility.
**Option C:** Milrinone is a phosphodiesterase inhibitor that increases cAMP levels, leading to an increase in cardiac contractility and relaxation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that inotropic agents can have varying effects on cardiac output and peripheral resistance, and their use should be carefully considered based on individual patient needs.

**Correct Answer:** D.