A 30-year-old man presents with a 3-week history of fatigue, occasional fever, yellow skin and sclerae, tenderness below the right costal margin, and dark urine. Physical examination reveals jaundice and mild hepatomegaly. Laboratory studies show elevated serum levels of bilirubin, decreased albumin, and prolonged prothrombin time. Serologic tests disclose antibodies to hepatitis C virus. Which of the following tests is the most accurate method for assessing the extent of liver disease in this patient?