A 65-year-old woman has chronic diarrhea for 5 months. Her stool is bulky and floats in the toilet bowel water. She also reports a 6kg weight loss. She has Hashimoto’s thyroiditis and takes levothyroxine. On examination, vital signs are normal. BMI is 20. Abdominal examination is normal. CBC reveals a microcytic hypochromic anemia. The serum ferritin level is decreased. A stool sample is negative for occult blood. Which of the following is the most appropriate diagnostic test to perform next?