Which of the following increases heart irritability?

The correct answer is hypercalcemia. Increased levels of calcium in the blood can lead to enhanced excitability of cardiac tissues. Calcium plays a critical role in the contractile process of the heart muscle, and an excess of calcium ions in the extracellular fluid affects the depolarization of cardiac cells. This can make the cardiac cells more sensitive to stimuli, thereby increasing heart irritability. When hypercalcemia occurs, it can lead to elevated myocardial contractility and potentially arrhythmias due to the heightened responsiveness of the heart's electrical conduction system.

On the other hand, hypokalemia causes a decrease in heart irritability. Low potassium levels can lead to a reduced ability of cardiac cells to repolarize effectively, which decreases their responsiveness to stimuli, making the heart less irritable. Hyperkalemia generally has the opposite effect from hypercalcemia, ultimately leading to reduced excitability due to its effects on resting membrane potential. Hypocalcemia, which involves low calcium levels, would also lead to decreased contractility and irritability of the heart, similar to the effects of hypokalemia. Hence, while both hypokalemia and hypocalcemia decrease heart irritability, hypercalcemia specifically increases it.