Because aminotransferases and alkaline phosphatase are included in laboratory test panels, abnormalities are often detected in patients without signs or symptoms of liver disease. Mild elevations in serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) usually require retesting and resolve in one-third of cases. If abnormalities persist in subsequent testing, further evaluation is needed. All patients with persistent abnormalities in serum AST or ALT should be evaluated for which of the following?