This picture shows a red and swollen thigh and leg caused by a blood clot (thrombus) in the deep veins in the groin (iliofemoral veins) which prevents normal return of blood from the leg to the heart. Update Date: 05/04/2008Linda Vorvick, MD, Seattle Site Coordinator, Lecturer, Pathophysiology, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.