Review Question - QID 109806

A 62-year-old man presents to his primary care provider complaining of abdominal pain. He reports a 6-month history of gradually progressive epigastric pain and 10 pounds of unexpected weight loss. He has also noticed that his skin feels itchier than usual. His past medical history is notable for gout, hypertension, and diabetes mellitus. He takes allopurinol, enalapril, and glyburide. He has a 10-pack-year smoking history and a distant history of cocaine abuse. His temperature is 100.1°F (37.8°C), blood pressure is 135/85 mmHg, pulse is 100/min, and respirations are 20/min. On exam, he has notable hepatomegaly and a palpable gallbladder. A right upper quadrant ultrasound reveals an irregular extrahepatic mass originating from the gallbladder wall. Which of the following serum markers is most likely elevated in this patient?