**Core Concept**
Digitalis is a cardiac glycoside used to treat heart failure and arrhythmias. It works by inhibiting the **Na+/K+ ATPase pump**, leading to increased intracellular calcium and enhanced contractility. This mechanism has distinct effects on the electrocardiogram (ECG).

**Why the Correct Answer is Right**
The correct answer is related to the effects of digitalis on the ECG, which include a **decrease in the conduction velocity** through the atrioventricular (AV) node, resulting in a **prolonged PR interval**. Digitalis also causes an **increase in the contractility** of the heart, which can lead to a **shortening of the QT interval**. However, the most characteristic effect is the presence of **digitalis effect** on the ECG, which includes **ST segment depression** with a **sagging or scooped appearance**, often referred to as a "digitalis effect" or "reverse tick" sign.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because digitalis does not typically cause an increase in the QRS complex duration.
**Option B:** This option is incorrect because digitalis does not characteristically cause a prolongation of the QT interval.
**Option D:** This option is incorrect because digitalis can cause various changes, but the specific change mentioned in this option is not the most characteristic one.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that digitalis effect on the ECG can be seen even at therapeutic doses, and it is essential to distinguish this from toxicity, which can present with more severe arrhythmias and higher degrees of AV block.

**Correct Answer:** C. Shortened QT interval with ST segment depression.