A 13-year-old boy has had abdominal pain, bloating, nausea, liquid fatty gray stool with putrid smell for the last 3 years. Palpation reveals epigastric tenderness, as well as tenderness in the Desjardins' pancreatic point, Chauffard's triangle; there is positive Mayo-Robson's sign. Failure of exocrine pancreatic function has been suspected. What is the most informative method for evaluating the state of exocrine pancreatic function?