A 56-year-old woman has an 8-month history of mid-epigastric abdominal pain and diarrhea. The pain is worse after eating. She has 6-8 movements daily, which usually occur after a meal and has lost 6.8 kg over the past 6 months. On examination, she is afebrile and other vital signs are normal. BMI is 21. Abdominal exam discloses epigastric tenderness but no evidence of hepatosplenomegaly or masses. Rectal examination is negative for occult blood. Her investigations show normal serum AST, ALT, and high serum lipase level. Which of the following test is most likely to establish the diagnosis?