**Core Concept**
Digitalis, a cardiac glycoside, increases the force of contraction and contractility of the heart muscle by inhibiting the Na+/K+-ATPase pump. This leads to an increase in intracellular calcium, which in turn enhances the contractile force of the heart. However, digitalis decreases myocardial oxygen demand by reducing heart rate and contractility at high concentrations, which is a counterintuitive effect.

**Why the Correct Answer is Right**
Digitalis decreases myocardial oxygen demand by two mechanisms. First, it reduces the heart rate, which decreases the frequency of contractions and subsequently reduces the oxygen demand. Second, at high concentrations, digitalis decreases contractility, which reduces the energy required for contraction and subsequently reduces oxygen demand. This is in contrast to other inotropes, which increase myocardial oxygen demand by increasing contractility.

**Why Each Wrong Option is Incorrect**
**Option A:** The force of contraction is actually increased by digitalis, not decreased.
**Option B:** Myocardial contractility is increased by digitalis, not decreased.
**Option D:** Cardiac output is actually decreased by digitalis, especially at high concentrations, due to its negative inotropic effect.

**Clinical Pearl / High-Yield Fact**
Digitalis has a biphasic effect on the heart, with low concentrations increasing contractility and high concentrations decreasing it. This biphasic effect is due to the different binding sites of digitalis on the Na+/K+-ATPase pump.

✓ Correct Answer: C. Myocardial oxygen demand