Low venous return can be caused by which of the following factors?

Low venous return occurs when the amount of blood returning to the heart decreases, which can significantly affect cardiac output and overall circulation. One key factor that can lead to low venous return is obstructed venous return. This obstruction can arise from various conditions such as thrombus formation (blood clots), external pressure from tumors or enlarged organs, or even mechanical problems like compression from clothing or body positioning. When blood flow in the veins is blocked or restricted, it diminishes the volume of blood that can return to the heart, which can lead to symptoms like swelling and decreased circulation.

In contrast, while the other factors mentioned can influence cardiovascular dynamics, they do not directly lead to reduced venous return in the same manner. Excessive hydration generally increases blood volume, potentially improving venous return. Increased blood viscosity can affect circulation efficiency but is less likely to cause obstruction in venous flow directly. High systemic vascular resistance may impact arterial pressure and flow but does not directly obstruct the veins' ability to return blood to the heart.