A 54-year-old woman had fatigue for 8 months and dark urine which noticed 6 weeks ago. Positive physical examination findings include scleral icterus, multiple spider telangiectasias over the chest and back, a liver edge palpated 4 cm below the right costal margin, and an enlarged spleen palpated 6 cm below the left costal margin. Results of complete blood count as well as measurement of serum electrolytes, urea nitrogen, and creatinine are normal. Other laboratory tests show: Alanine aminotransferase 78 U/L (10 - 40) Aspartate aminotransferase 680 U/L (10 - 30) Serum bilirubin 20.8 mg /dl (0.3 - 1.2) Direct bilirubin 8.7 mg/dl (0 - 0.2) Alkaline phosphatase 145 U/L (25 - 100) Serum albumin 2.8 g/dl (3.4 - 4.8) Serum protein 8.7 g/dl (6.4 - 8.3) Prothrombin time 12 sec (18 - 22) Which of the following diagnostic studies should be performed next?