Final answer:
Lymph nodes filter the fluid draining from the legs, feet, and groin, being a major part of the lymphatic system that helps in immune defense by trapping and destroying pathogens.
Step-by-step explanation:
The organ that filters fluid draining from the legs, feet, and groin is the lymph nodes. Lymph nodes are integral to the lymphatic system, which serves as a critical component of the body's immune defense. These small, bean-shaped structures are situated along the network of lymphatic vessels and act as biological filters. They work to remove debris, pathogens, and cancer cells from the lymph fluid before it is returned to the bloodstream. Lymph nodes play a crucial role in the body's ability to fight infections since any bacteria or other microorganisms present in the interstitial fluid from the legs and other areas are captured and destroyed by lymphocytes within these nodes.