MRCP2-1980

You evaluate a 35-year-old man at the Dermatology Clinic who presents with unsightly pustules on his shin that have formed an ulcerated patch and appear blue-black. His General Practitioner suspects a spreading bacterial infection. The patient has a history of diarrhoea but no other significant medical history. Blood tests show elevated viscosity, C-reactive protein, and alkaline phosphatase. A biopsy of the ulcer edge reveals vasculitis and an intense neutrophilic infiltrate. What is the most probable underlying cause of this patient’s skin changes and symptoms?