**Question:** All of the following statements about digitalis are true except:

A. Digitalis is used to treat congestive heart failure (CHF).
B. Digitalis interferes with the release of calcium ions from sarcoplasmic reticulum.
C. Digitalis selectively targets type II sodium channels in neurons.
D. Digitalis competes with ouabain for binding to the sodium pump.

**Core Concept:** Digitalis is a cardiac glycoside that primarily works by inhibiting the sodium-potassium pump (Na+/K+-ATPase) in the plasma membrane of cardiac myocytes, leading to increased intracellular sodium and calcium ions, which results in increased cardiac contractility and decreased heart rate (bradycardia). Digitalis is commonly used in the management of congestive heart failure (CHF).

**Why the Correct Answer is Right:**
Digitalis is used for the treatment of CHF, which is primarily due to its effect on cardiac myocytes and its ability to increase contractility and decrease heart rate. However, option C ("Digitalis selectively targets type II sodium channels in neurons") is incorrect because digitalis does not target neurons directly but affects cardiac myocytes.

**Why Each Wrong Option is Incorrect:**
A. While digitalis does contribute to CHF treatment, its primary mode of action is on cardiac myocytes, making this statement misleading regarding its direct therapeutic effect.
B. Although digitalis does inhibit Na+/K+-ATPase, the explanation provided in option D ("Digitalis competes with ouabain for binding to the sodium pump") is more accurate than option B, which does not explain the mechanism.
C. Digitalis does affect cardiac myocytes, but it does not selectively target type II sodium channels in neurons, making this statement false.

**Why the Correct Answer is Right (D):**
Digitalis competes with ouabain for binding to the sodium pump (Na+/K+-ATPase), leading to increased intracellular sodium ions. This increase in sodium ions causes an increase in calcium ions, which in turn enhances cardiac contractility and reduces heart rate (bradycardia). The primary therapeutic effect is on cardiac myocytes, not neurons.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect due to the same reason as the correct answer: the primary therapeutic effect is on cardiac myocytes, not neurons.
B. This explanation is more accurate than option B and clearly describes the mechanism of action of digitalis.
C. This option is incorrect because digitalis does affect cardiac myocytes, but it does not selectively target type II sodium channels in neurons.

**Clinical Pearl:**
Understanding the mechanism of action of digitalis is essential for its proper use in the management of CHF. The therapeutic effect on cardiac myocytes is crucial, as it improves cardiac contractility and reduces heart rate, which is beneficial in CHF. However, it is essential to differentiate between the direct effects on cardiac myocytes and indirect effects on neurons, as they are different.