High levels of magnesium can lead to which of the following physiological changes?

High levels of magnesium, also known as hypermagnesemia, can indeed lead to hypotension as a significant physiological change. Magnesium functions as a natural calcium antagonist and can cause vasodilation. This vasodilation leads to a decrease in systemic vascular resistance, which can subsequently lower blood pressure and result in hypotension. Additionally, elevated magnesium levels can slow down the conduction through the heart, which may contribute to various cardiac issues, but the direct and notable effect in this case is the reduction of blood pressure.

While cardiac arrhythmias can occur due to hypermagnesemia, the central and immediate consequence related to hypotension from high magnesium levels makes this the most appropriate selection. Other options such as tachycardia and hyperthermia are not typical effects of high magnesium levels. In fact, tachycardia is more often associated with electrolyte imbalances like hypokalemia, whereas hyperthermia is not a recognized result of elevated magnesium. Therefore, hypotension stands out as the primary physiological change directly related to high levels of magnesium.