Which of the following is least likely to occur as a side effect of magnesium sulfate used in preterm labor?

The correct answer is hypertension, which is least likely to occur as a side effect of magnesium sulfate used in preterm labor. Magnesium sulfate is primarily utilized for its neuroprotective effects on the fetus and as a tocolytic agent to delay preterm labor. One of its pharmacological effects is the relaxation of vascular smooth muscle, which typically leads to a decrease in blood pressure, making hypotension, flushing, and headache more common side effects.

Hypotension can occur due to the vasodilatory effects of magnesium sulfate, leading to decreased peripheral vascular resistance and potentially resulting in lower blood pressures. Flushing is also a common side effect due to increased blood flow to the skin as a result of vascular dilation. Headaches may arise from various factors, including changes in blood pressure and vascular effects, hence it can also be seen as a response to the medication.

In contrast, hypertension is not an expected side effect of magnesium sulfate administration. It generally does not cause increased blood pressure and, in fact, the opposite is observed in most cases. Thus, out of all the listed side effects, hypertension is the least likely to occur when magnesium sulfate is used in a clinical setting for preterm labor.