## **Core Concept**
The question tests the understanding of the mechanism of action of digitalis, specifically how it exerts its positive inotropic effect on cardiac muscle cells. Digitalis acts by inhibiting the **Na+/K+ ATPase pump** (also known as the sodium-potassium pump) in the cardiac muscle cell membrane. This pump is crucial for maintaining the resting potential and regulating intracellular sodium and potassium levels.

## **Why the Correct Answer is Right**
The inhibition of the Na+/K+ ATPase pump by digitalis leads to an increase in intracellular sodium levels. This increase in intracellular sodium triggers a cascade of events. The **sodium-calcium exchanger** (NCX), which is present in the cardiac muscle cell membrane, works to extrude excess sodium. As a result, there is an increase in intracellular calcium levels because the NCX exchanges one calcium ion for one sodium ion moving into the cell. The increased intracellular calcium enhances the contractility of the cardiac muscle by increasing the amount of calcium available for binding to troponin and tropomyosin, thus initiating muscle contraction. This mechanism explains the positive inotropic effect (increased contractility) of digitalis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because while digitalis does affect potassium levels by inhibiting the Na+/K+ ATPase pump, the direct positive inotropic effect is not primarily due to changes in potassium levels but to the increase in intracellular calcium.
- **Option B:** Incorrect because the decrease in intracellular sodium is not what leads to the positive inotropic effect; rather, it's the increase in intracellular sodium that triggers the cascade leading to increased intracellular calcium.
- **Option C:** (Correct Answer) Not explained here as per instructions.
- **Option D:** Incorrect because the direct effect of digitalis on the **Na+/K+ ATPase pump** inhibition leads to increased intracellular sodium, not decreased, which then indirectly increases intracellular calcium.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that digitalis toxicity can occur due to inhibition of the Na+/K+ ATPase pump in other tissues, leading to **hyperkalemia** (high potassium levels) and **arrhythmias**. Monitoring potassium levels and renal function is crucial when administering digitalis.

## **Correct Answer:** . Increased intracellular calcium.

✓ Correct Answer: A. Decreased activity of Na+/Ca++ exchanger causing decreased influx of sodium and decreased efflux of Ca++ in the sarcolemma