Digitalis is a medication that increases stroke volume by increasing intracellular ______________.

Digitalis, also known as digoxin, primarily works by increasing the intracellular concentration of calcium ions (Ca2+). This increase in intracellular calcium leads to enhanced contractility of the cardiac muscle, which results in an increased stroke volume. The mechanism involves inhibition of the Na+/K+ ATPase pump in the cardiac cell membrane. When this pump is inhibited, there is a subsequent increase in intracellular sodium (Na+). The elevated sodium levels then influence the sodium-calcium exchange mechanism, leading to an influx of calcium into the cells. This increase in calcium available for myocardial contraction directly enhances the force of the heart's contractions, thus increasing stroke volume.

It is essential to understand that while sodium plays a role in the mechanism, it is primarily the elevation of calcium levels that directly leads to improved cardiac contractility. Therefore, the correct answer focuses on calcium as the critical intracellular ion that affects stroke volume in the context of digitalis action.