Which of the following can result in long QT intervals?

The correct choice is hypocalcemia, as low levels of calcium in the blood can lead to a prolonged QT interval on an electrocardiogram (ECG). Calcium plays a crucial role in cardiac electrical activity, particularly in the repolarization phase of the cardiac cycle. When calcium levels are low, it can disrupt normal cardiac rhythm and prolong the QT interval, which increases the risk for potentially dangerous arrhythmias.

In contrast, conditions such as hypercalcemia generally have the opposite effect, often shortening the QT interval due to increased calcium affecting cardiac muscle contractility and the rate of repolarization. Abnormal potassium levels also influence the QT interval but typically present differently; hyperkalemia tends to cause a shortening of the QT interval. Normal calcium levels would not result in changes to the QT interval, and therefore would not contribute to prolonged QT in the way hypocalcemia does. Therefore, hypocalcemia is the most relevant condition linked to the prolongation of the QT interval among the presented options.