For the last 3 years a 12-year-old boy has been suffering from stomachache, abdominal distension, nausea, periodical liquid fatty stool, grey in color, with rotten smell. On palpation: pain in the epigastrium, Desjardins' pancreatic point and Chauffard's triangle; positive Mayo-Robson's sign. Insufficiency of pancreas exocrine function is suspected. What method is the most informative for pancreas exocrine function assessment?