A 52-year-old man presents to the emergency department with complaints of abdominal pain, progressive abdominal swelling, and fatigue over the past two weeks. He reports increasing difficulty breathing when lying flat and unintentional weight gain due to fluid retention. He denies fever, nausea, or significant alcohol consumption but mentions a history of intermittent headaches and dizziness. On physical examination, he is noted to have hepatomegaly, abdominal distension with shifting dullness, and mild jaundice. His blood pressure and heart rate are normal, but his oxygen saturation is slightly reduced in the supine position. Liver function tests show mild transaminase elevations and hypoalbuminemia, while abdominal ultrasound with Doppler confirms hepatic vein thrombosis, consistent with Budd-Chiari syndrome (BCS). Of the following, which is the most frequent underlying disease associated with Budd-Chiari syndrome?
