**Core Concept**
Ouabain is a cardiac glycoside that plays a crucial role in the regulation of cardiac function. It acts by inhibiting a key enzyme involved in the sodium-potassium pump, which is essential for maintaining the normal electrical activity of the heart.

**Why the Correct Answer is Right**
Ouabain specifically inhibits the Na+/K+-ATPase enzyme, which is responsible for the sodium-potassium pump. This pump is crucial for maintaining the resting membrane potential of cardiac cells. By inhibiting this enzyme, ouabain increases the intracellular sodium concentration, which in turn increases the intracellular calcium concentration. This increase in calcium leads to an increase in the contractility of the heart muscle. The Na+/K+-ATPase enzyme is a transmembrane enzyme that uses ATP to pump sodium ions out of the cell and potassium ions into the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ouabain does not act by inhibiting the calcium channels in the heart.

**Option B:** Incorrect because ouabain does not act by inhibiting the potassium channels in the heart.

**Option C:** Incorrect because ouabain does not act by inhibiting the sodium channels in the heart.

**Clinical Pearl / High-Yield Fact**
Ouabain is a classic example of a cardiac glycoside that can be used to increase the contractility of the heart muscle, but it must be used with caution due to its potential for toxicity.

**Correct Answer: D. Na+/K+-ATPase enzyme**