Which of the following best describes a vasopressor?

A vasopressor is best defined as a drug that results in arteriolar constriction. This action is critical in the management of conditions such as hypotension, where increasing vascular resistance is necessary to raise blood pressure. By constricting blood vessels, vasopressors effectively increase systemic vascular resistance, which can improve perfusion to vital organs in states of shock or severe hypotension.

The other options do not accurately describe the primary action of a vasopressor. For instance, a medication that causes vasodilation (the first option) would lower blood pressure instead of raising it, which directly contradicts the function of a vasopressor. Treatment for fluid overload (the third option) typically involves diuretics or other interventions to manage excess fluid, rather than affecting blood vessel constriction. Lastly, while enhancing oxygenation (the fourth option) is an important aspect of critical care, it is not the primary function of a vasopressor. Vasopressors do not directly improve oxygenation; instead, their role is focused on stabilizing blood pressure and improving perfusion.