A 50-year-old woman is brought to the emergency department because of severe upper abdominal pain for 24 hours. She has a 2-day history of intermittent fever and pale stools. Her temperature is 38.9°C (102°F), pulse is 120/min, respirations are 20/min, and blood pressure is 90/50 mm Hg. Physical examination shows jaundice and tenderness of the right upper quadrant of the abdomen. Serum studies show a bilirubin concentration of 5 mg/dL, alkaline phosphatase activity of 450 U/L, and lipase activity of 400 U/L (N=14–280). Which of the following is the most likely cause of this patient’s symptoms?