What fluid other than blood is used to drain excess tissue fluid?

Lymph is the correct answer because it is a clear fluid that circulates throughout the lymphatic system and plays a crucial role in maintaining fluid balance in the body. It collects excess interstitial fluid, which is the fluid that exists in the spaces between cells, thereby preventing tissue swelling and helping to ensure that the environment around cells remains stable. Lymphatic vessels transport this fluid back into the bloodstream, effectively draining excess tissue fluid.

The other options, while related to bodily fluids, do not serve the same function as lymph does in terms of draining excess tissue fluid. Urine is a waste product that is excreted from the body and is not involved in fluid balance in tissues. Serum, which is the liquid part of blood after clotting, contains nutrients and proteins but is not responsible for draining tissue fluid. Interstitial fluid is the same as the excess fluid that lymph collects and transports, meaning that it cannot be considered a draining fluid itself.