A 36-year-old woman has become increasingly icteric for 1 month. She has had several bouts of colicky, midabdominal pain for 3 years. On physical examination, she has generalized jaundice with scleral icterus. Her BMI is 32. There is tenderness in the right upper quadrant, and the liver span is normal. A liver biopsy is obtained, and microscopic examination shows bile duct proliferation and intracanalicular bile stasis, but no inflammation or hepatocyte necrosis. The level of which of the following is most likely to be increased in the patient's serum?