A 48-year-old woman presents to the clinic with a 6-month history of intense pruritus and fatigue. She reports dark urine and pale stools over the past several weeks. Physical examination reveals yellowing of the sclera, multiple xanthelasmas on her eyelids, and mild hepatomegaly. Laboratory studies show: alkaline phosphatase 560 U/L (normal 44–147), ALT 85 U/L (normal 7–56), AST 78 U/L (normal 10–40), total bilirubin 3.2 mg/dL (normal 0.1–1.2). Serum immunology reveals antimitochondrial antibody (AMA) positivity and elevated IgM levels. A right upper quadrant ultrasound is unremarkable for biliary dilation. Which of the following is the most likely diagnosis?