A 38-year-old man presents with jaundice, abdominal pain, and weight gain 14 days after undergoing hematopoietic stem cell transplantation (HSCT) for aplastic anemia. On examination, he has right upper quadrant tenderness, hepatomegaly, and ascites. Laboratory findings show a total bilirubin of 3.2 mg/dL, mild transaminase elevation, and prolonged INR. Doppler ultrasound reveals reduced hepatic venous flow with no thrombosis. What is the most likely diagnosis?
