**Core Concept**
Digitalis, a cardiac glycoside, is used in the treatment of heart failure and atrial fibrillation. Its mechanism of action is primarily related to its inhibition of the sodium-potassium ATPase pump, also known as the Na+/K+-ATPase pump. This inhibition leads to an increase in intracellular sodium levels, which in turn increases intracellular calcium levels through the sodium-calcium exchange mechanism.

**Why the Correct Answer is Right**
The increased intracellular calcium levels enhance the contractility of the myocardium, thereby improving cardiac output in heart failure patients. Additionally, digitalis increases the refractory period of the atrioventricular (AV) node, which is beneficial in the treatment of atrial fibrillation by slowing the heart rate and allowing for more efficient ventricular filling. The increased intracellular calcium levels also lead to a decrease in the rate of diastolic depolarization in the AV node, further contributing to the reduction in heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because digitalis does not act by directly increasing the force of cardiac contractions through a positive inotropic effect, but rather by increasing the sensitivity of the myocardium to calcium ions.
**Option B:** This option is incorrect because digitalis does not act by directly inhibiting the sympathetic nervous system. While digitalis can lead to a decrease in heart rate, this is not due to a direct action on the sympathetic nervous system, but rather through its effects on the AV node.
**Option C:** This option is incorrect because digitalis does not act by directly increasing the levels of cyclic adenosine monophosphate (cAMP) in cardiac cells. While cAMP is an important second messenger in cardiac cells, digitalis's mechanism of action is primarily related to its effects on the sodium-potassium ATPase pump.

**Clinical Pearl / High-Yield Fact**
One of the classic exam traps related to digitalis is the concept of "digitalis toxicity," which can occur when the levels of digitalis in the bloodstream are too high. Digitalis toxicity is characterized by symptoms such as nausea, vomiting, and arrhythmias, and can be life-threatening if not treated promptly.

**Correct Answer:** C. Inhibition of the sodium-potassium ATPase pump.