**Core Concept:** Digoxin is a cardiac glycoside used in the management of heart failure and atrial fibrillation. It works by increasing the contractility of cardiomyocytes, reducing the heart rate, and decreasing the workload on the heart.

**Why the Correct Answer is Right:** Digoxin is a cardiotonic agent that enhances the contractility of cardiac muscle cells (cardiomyocytes) by increasing the sensitivity of the sarcoplasmic reticulum calcium ATPase pump to calcium ions, which leads to increased calcium uptake and improved contractility. This helps to reduce the heart rate and decrease the heart's workload.

**Why Each Wrong Option is Incorrect:**

A. **False:** Digoxin is not primarily an antiplatelet agent nor does it prevent platelet aggregation. Its primary action is on cardiomyocytes, as mentioned above.

B. **False:** Digoxin does not directly increase cardiac output by acting on the sympathetic nervous system. Its effect is on the cardiomyocytes, specifically enhancing contractility, thus indirectly improving cardiac output.

C. **False:** Digoxin does not act as a vasodilator. It decreases the workload on the heart by improving contractility and heart rate, which indirectly improves cardiovascular efficiency and reduces afterload.

D. **False:** Digoxin does not cause hypokalaemia as a direct consequence of its use. Hypokalaemia is a potential side effect due to the inhibition of the Na+/K+-ATPase pump in the kidney, leading to potassium loss from the body fluids.

**Clinical Pearl:** Monitoring serum potassium levels is essential when initiating digoxin therapy to prevent hypokalaemia-induced side effects, such as muscle weakness, arrhythmias, and paralysis. Promptly addressing hypokalaemia helps ensure safe and effective digoxin therapy.

**Correct Answer:** B. Digoxin does not directly increase cardiac output by acting on the sympathetic nervous system. Its effect is on the cardiomyocytes, specifically enhancing contractility, thus indirectly improving cardiac output.