A 56-year-old man presents to his internist with jaundice. The patient is receiving no medication and his only symptomatic complaint is mild fatigue over the past 2 months. Physical examination is remarkable only for the presence of scleral icterus. The patient has no significant past medical history. Analysis of serum chemistry reveals the following- SGOT : 0.58 microkat/L (35U/L) SGPT : 0.58 microkat/L (35U/L), Total bilirubin: 91.7 micron mol/L (7mg/dL) Direct Bilirubin:85.5 micron mol/L (5 mg/dL) Alkaline phosphatase : 12 micron kat/L(720 U/L) Which of the following is the next most appropriate diagnostic step?