## Core Concept
The primary action of drugs like digoxin, which increases cardiac contractility, involves the inhibition of the sodium-potassium ATPase pump in cardiac cells. This action leads to an increase in intracellular calcium levels, enhancing the contractility of the heart. Digoxin is a cardiac glycoside used in the treatment of heart failure and certain arrhythmias.

## Why the Correct Answer is Right
The correct answer, **B. Inhibition of Na+/K+ ATPase**, is right because digoxin works by inhibiting the sodium-potassium ATPase pump (Na+/K+ ATPase) on the cardiac cell membrane. This inhibition leads to an accumulation of sodium inside the cell. The increased intracellular sodium concentration indirectly leads to an increase in intracellular calcium through the sodium-calcium exchanger. The elevated intracellular calcium levels increase the contractility of the heart by sensitizing the cardiac myofilaments to calcium and by increasing the amount of calcium available for binding during systole.

## Why Each Wrong Option is Incorrect
- **Option A: Direct activation of adenylate cyclase** is incorrect because while increasing cyclic AMP (cAMP) levels can increase cardiac contractility (e.g., through beta-adrenergic receptor agonists), digoxin's primary mechanism does not involve direct activation of adenylate cyclase.
- **Option C: Blockade of potassium channels** is incorrect because, although digoxin does affect potassium levels (patients on digoxin can exhibit hypokalemia, which can increase the risk of digoxin toxicity), its primary action to increase contractility is not through blockade of potassium channels.
- **Option D: Stimulation of the parasympathetic nervous system** is incorrect because, while digoxin does have effects on the autonomic nervous system (it can increase vagal tone, which is part of its anti-arrhythmic effect), its primary action to increase cardiac contractility is not through stimulation of the parasympathetic nervous system.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the **therapeutic effect of digoxin is highly dependent on its serum concentration**, and monitoring potassium levels is crucial because **hypokalemia can increase the risk of digoxin toxicity**. This is a high-yield fact for exams and clinical practice.

## Correct Answer: B. Inhibition of Na+/K+ ATPase.