**Question:** The biochemical mechanism of action of digitalis is associated with

A. inhibition of Na+/K+-ATPase
B. increase in intracellular calcium
C. direct stimulation of cardiac muscle contraction
D. inhibition of the respiratory chain

**Core Concept:** Digitalis is a group of cardiac glycosides derived from the foxglove plant (Digitalis purpurea). It has a long history of use in the treatment of heart failure and arrhythmias by increasing the contractility of the heart muscle (cardiac glycosides). The primary target of digitalis is the Na+/K+-ATPase pump, which plays a crucial role in maintaining the electrochemical gradient of sodium and potassium ions across the cell membrane.

**Why the Correct Answer is Right:** The correct answer, A, highlights the key action of digitalis on the Na+/K+-ATPase pump. Na+/K+-ATPase is a membrane-bound enzyme responsible for maintaining the electrochemical gradient of sodium and potassium ions, which is essential for the proper functioning of the heart muscle. By inhibiting this pump, digitalis increases the intracellular calcium concentration, leading to enhanced contractility and improved cardiac output.

**Why Each Wrong Option is Incorrect:**

B. Inhibition of intracellular calcium is not the primary mechanism of action of digitalis. Instead, it enhances the contractility of the heart muscle by affecting the Na+/K+-ATPase pump and calcium dynamics.

C. Digitalis does not directly stimulate cardiac muscle contraction; rather, it improves contractility through the mentioned mechanism involving Na+/K+-ATPase.

D. Digitalis does not inhibit the respiratory chain, which is unrelated to its mechanism of action on cardiac muscle contractility.

**Clinical Pearl:** Understanding the mechanism of action of digitalis is crucial for its appropriate use in patients with heart failure and arrhythmias. However, digitalis therapy should be used cautiously due to its narrow therapeutic window, potential toxicity, and drug interactions. Clinical judgement and regular monitoring of serum digitalis levels are essential for optimal patient outcomes.