Q. A 35-year-old man comes to the emergency department because of a month-long history of fatigue, yellowing of his skin, and pruritus. He reports episodes of bloody diarrhea over the past year, but he did not seek a medical evaluation for it. He has a history of heroin injection when he was a teenager but he has not used heroin in over 15 years. He has smoked one pack of cigarettes a day for the past 20 years. His vitals are normal. On examination, he has hepatomegaly. A colonoscopy reveals inflamed mucosa in the rectum. Labwork is presented in the table below. Which of the following is the gold standard in diagnosing this disorder?