A 56 year old man presents to his internist with jaundice. The patient is receiving no medication and his only symptomatic complaints 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 (7 mg/dL) Direct Bilirubin : 85.5 micron mol/L (5mg/dL) Alkaline phosphatase:12 micron kat/L (720U/L) Which of the following is the next appropriate diagnostic step?