A 56-year-old man is evaluated for recent onset painful skin lesion which involves his abdominal wall. The lesion started 3 days ago as a small erythematous macule which has gradually increased in size to a large purpuric lesion with bulla formation. He is afebrile and does not recall any trauma. His medical history is significant for atrial fibrillation; he was recently switched from rivaroxaban to warfarin due to the high cost of rivaroxaban.What is the most probable cause of the condition?