A 26-year-old female presents with low-grade fever, fatigue, and night sweats for 1 month. Physical exam: alert, weight loss, jugular venous distension in supine position, 2/6 systolic blowing murmur at the apex. Distended abdomen with doughy consistency, mild generalized tenderness and rebound tenderness, liver palpable 2.5 cm below the costal margin (blunt edge, medium consistency, tender (+)), shifting dullness (+), no lower limb edema. ALT 56 IU/L, total bile acids 8 μmol/L, urinary protein (+). Which test is the first choice for definitive diagnosis?