52-yo woman with a history of Hashimoto thyroiditis developed signs of biliary obstruction. Malignancy was suspected. Imaging studies showed mild diffuse enlargement of pancreas and segmental dilation of main pancreatic duct. No discreet mass was detected. What additional tests would be valuable in diagnostic workup?
Answer: "Serum IgG4 level, biopsy of the pancreas, brief steroid challenge."