Which condition would lead to decreased preload and subsequently lower stroke volume?

Dehydration leads to decreased preload due to the reduction in blood volume. Preload is defined as the volume of blood in the ventricles at the end of diastole, which sets the stage for the amount of blood ejected during systole (stroke volume). When a person is dehydrated, there is less fluid in the circulatory system, which lowers the overall blood volume returning to the heart. This decrease in return flow reduces the stretch on the heart muscle prior to contraction, ultimately resulting in lower stroke volume. Therefore, dehydration effectively diminishes the preload due to the reduced venous return, causing the heart to pump less blood with each beat.

In contrast, exercise generally increases preload by enhancing venous return through increased heart rate and muscle contractions that facilitate blood flow. Increased blood volume would naturally elevate preload and stroke volume, while systemic vasodilation can cause an increase in effective blood flow, which can also enhance preload under certain circumstances. Thus, among the choices provided, dehydration is the condition that most directly leads to decreased preload and subsequently lowers stroke volume.