Which anesthetic agent can cause a fall in afterload, potentially leading to hypotension?

The correct answer is nitroglycerin. Nitroglycerin is a vasodilator that acts primarily on the venous system, leading to a decrease in preload and potential reduction in afterload. By dilating blood vessels, nitroglycerin lowers vascular resistance, which can cause a fall in afterload and potentially lead to hypotension, especially in patients who are already compromised or volume-depleted.

Norepinephrine is a vasopressor and typically increases blood pressure by raising vascular tone and increasing systemic vascular resistance, which is the opposite effect. Ketamine, while it has various effects on the cardiovascular system, is generally considered to have minimal effects on blood pressure and may even cause a rise in blood pressure due to sympathetic stimulation in certain clinical scenarios. Fentanyl, an opioid, may lead to hypotension in some cases, but its mechanism is not primarily through vasodilation or changes in afterload; it can cause bradycardia or respiratory depression, which can also contribute to hypotension.

Thus, the option of nitroglycerin stands out as the agent specifically known for its effect on vascular dilation, leading to decreased afterload and a potential drop in blood pressure.