EC injury in a case of antiphospholipid antibody syndrome (photographs courtesy of Emma E. Furth, MD, Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA). EC injury seen in a duodenal biopsy from a 40-year-old woman who presented with profuse intestinal bleeding and was found to have a lupus anticoagulant and a markedly positive anticardiolipin antibody. (A) Elastic stain (original magnification × 200) highlighting a fresh thrombus (right) with the beginning stages of organization and EC ingrowth, an older organized thrombus with fibroblast proliferation (center), within a vessel showing vacuolated, injured, and disrupted ECs. (B) A trichrome stain (original magnification × 400) of the same field highlighting the thrombus material (red acellular material on the right) with early stages of organization. (C) A hematoxylin and eosin (H and E) stain (original magnification × 400) showing fibrinoid intimal necrosis (right) in the absence of an inflammatory reaction within a small vessel in the same duodenal biopsy as shown in (A) and (B). The ECs show marked vacuolization (left). The surrounding eosinophilic vascular cells are smooth muscle cells surrounded by fibroblasts. (D) A higher power view (original magnification × 600) of the same biopsy showing four capillaries with grossly vacuolated ECs and luminal effacement.