A 50-year-old man with muscle pain and fever for a month now notes darker colored urine for the past 2 weeks. On physical examination, he has palpable purpuric lesions of his skin. Urinalysis shows hematuria and proteinuria. Serum laboratory findings include mixed cryoglobulinemia with a polyclonal increase in IgG, as well as a high titer of anti-neutrophil cytoplasmic autoantibodies, mainly antimyeloperoxidase (MPO-ANCA, or P-ANCA. A skin biopsy is performed. What pathologic finding is most likely to be observed in this biopsy?

Microscopic polyangiitis involves small vessels, typically capillaries. Kidneys and lungs are commonly involved, but many organs can be affected. There may be an underlying immune disease, chronic infection, or drug reaction. Giant cell arteritis typically involves arterial branches of the external carotid, most often the temporal artery. Micro abscesses may be present with an infectious process, or with thromboangiitis obliterans (Buerger disease), which typically involves lower extremities. Mycotic aneurysms occur when a focus of infection, often from a septic embolus, weakens an arterial wall so that it bulges out. Perivascular eosinophilic infiltrates may be seen with Churg-Strauss syndrome, which typically involves the lungs.