A 73-year-old woman has noticed a 10-kg weight loss in the past 3 months. She is becoming increasingly icteric and has constant vague epigastric pain, nausea, and episodes of bloating and diarrhea. On physical examination, she is afebrile. There is mild tenderness to palpation in the upper abdomen, but bowel sounds are present. Her stool is negative for occult blood. Laboratory findings include a total serum bilirubin concentration of 11.6 mg/dL and a direct bilirubin level of 10.5 mg/dL. Which of the following conditions involving the pancreas is most likely to be present?