1. A 50-year-old woman presents with abdominal pain and jaundice. Imaging reveals a segmentally enlarged pancreas with a “sausage-like” appearance. She undergoes a pancreatoduodenectomy. Microscopically, the tumor shows a duct-centric lymphoplasmacytic infiltrate and storiform fibrosis, with numerous plasma cells expressing IgG4. What is the most likely diagnosis?