What is the hallmark sign of thrombophlebitis?

Thrombophlebitis is characterized primarily by inflammation of a vein, which results in clinical manifestations such as redness, swelling, and tenderness along the affected area. The hallmark sign—specifically—involves these three components: inflammation, which is the body's response to injury; redness, due to increased blood flow and vascular permeability; and tenderness, a result of the irritation and inflammation of the venous wall.

In the context of thrombophlebitis, these signs signify that a thrombus, or blood clot, has formed in a superficial vein and the surrounding tissue is responding to the clot, manifesting these signs of inflammation. This is in contrast to the other symptoms listed in the question, which pertain to different medical conditions and do not directly indicate thrombophlebitis.