The build-up of lymph fluid in tissues is known as:

The build-up of lymph fluid in tissues is correctly identified as lymphedema. This condition occurs when the lymphatic system is compromised, which can lead to an abnormal accumulation of lymph fluid. This can result from various causes, such as surgical removal of lymph nodes, radiation therapy, infection, or congenital problems with the lymphatic system.

The term "erythema" refers to redness of the skin caused by increased blood flow to the capillaries, usually in response to injury or inflammation, which does not relate to lymph fluid accumulation. "Sepsis" is a severe response to infection that affects the whole body and is characterized by symptoms like fever and an increased heart rate; it does not pertain to lymphatic fluid. "Anorexia" is a condition characterized by a loss of appetite or desire to eat, and while it may have serious health implications, it is not associated with the accumulation of lymph fluid.

Therefore, lymphedema specifically describes the condition where lymph fluid builds up in tissues due to lymphatic obstruction or dysfunction.