An 80-year-old woman is evaluated for increasing somnolence for several hours. She has hypertension and diabetes. On examination, she is jaundiced, lethargic and disoriented. Her temp is 38.3, BP is 110/62mmHg, and pulse rate is 100/min. Her right upper quadrant is tender without guarding. She has no neurological deficit. Her investigations show leukocytosis, high alkaline phosphatase and total bilirubin. Abdominal US shows normal liver, dilated common bile duct of 9mm, multiple gallstones, but no evidence of gallbladder wall thickening. Which of the following is the most likely diagnosis?