Which of the following is a contraindication for digitalis toxicity?

Digitalis, commonly used in treating heart conditions, can lead to toxicity, especially if certain conditions are present. Calcium is particularly important in this context. An elevated calcium level, or hypercalcemia, can exacerbate the effects of digitalis, increasing the risk of digitalis toxicity. Digitalis already increases intracellular calcium levels in cardiac myocytes, enhancing contractility. Therefore, if a patient has elevated calcium levels, the potential for toxic effects increases significantly.

While hypoglycemia, dehydration, and hypotension may have implications for a patient’s overall health and could complicate a clinical picture, they do not have the same direct and potent interaction with digitalis as calcium does. For instance, hypoglycemia does not directly influence digitalis toxicity's mechanisms, while dehydration can actually increase the concentration of digitalis due to a decrease in blood volume. Hypotension is more about the patient’s hemodynamics rather than a direct contraindication.