Patient Case: A 60-year-old man presents with 3 days of jaundice, dark urine, and intermittent right upper quadrant (RUQ) pain. He denies fever or chills but reports a 2-week history of mild nausea and pruritus. Physical examination reveals scleral icterus and mild RUQ tenderness. Abdominal ultrasound shows a dilated common bile duct (CBD) measuring 10 mm but no gallstones in the gallbladder. Which of the following is the most likely cause of this patient’s symptoms?
