What can cause a prolonged QT interval in a patient?

A prolonged QT interval is primarily associated with disturbances in electrolyte levels and certain medications. In the context of the options provided, hypomagnesemia is a significant cause of a prolonged QT interval. Magnesium plays a critical role in myocardial repolarization, and low levels can lead to impaired electrical activity of the heart. This impairment can manifest as a lengthened QT interval on the electrocardiogram (ECG).

Low magnesium levels may also potentiate the effects of other electrolyte imbalances, notably hypokalemia, which can further complicate cardiac repolarization. The relationship between magnesium and potassium levels is important, as magnesium is required for the proper entry of potassium into cells, and deficits in magnesium can lead to potassium loss and further prolongation of the QT interval.

In contrast, while hyperkalemia and hypokalemia can have significant effects on the heart's electrical activity, especially in relation to the T wave, they typically do not directly cause a prolonged QT interval in the same way that hypomagnesemia does. Hypernatremia is not usually associated with changes in the QT interval; instead, it can affect fluid balance and vascular tone but does not have a direct impact on the cardiac repolarization processes that determine the QT interval duration