Polyarteritis nodosa (PAN) is an inflammatory necrotizing disease of the small and the medium sized arteries of the body. Histologically it is characterized by fibrinoid necrosis of the arterial wall with a leukocytic infiltrate. In this slide, there is a large pale occlusion of a muscular artery. Within this collagenous tissue is a leukocytic infiltrate which is the hallmark of PAN.