Maria, a 44-year-old woman with a history of paroxysmal nocturnal hemoglobinuria (PNH) and chronic alcohol use, presents with severe abdominal pain, abdominal distention, and shortness of breath over the past week. She also reports intermittent vomiting. Physical examination reveals jaundice, a protuberant abdomen with right upper quadrant tenderness, painful hepatomegaly extending 7 cm below the costal margin, and moderate ascites. Visible dilated abdominal veins are noted when she is standing, along with bilateral pitting edema. Her vital signs show mild hypotension (BP 87/57 mmHg) and tachycardia (HR 112 bpm). Given her history of hypercoagulable conditions and physical findings, you suspect hepatic vein thrombosis (Budd-Chiari syndrome). Of the following, which is the initial test of choice in this patient to confirm the diagnosis?
