A 35- year-old woman has a chronic diarrhea for 5 months. It is foul smelling, bulky, and float in toilet bowel water. It is associated with postprandial bloating and 3 kg weight loss. She has iron deficiency anemia and Hashimoto’s thyroiditis. Her family history is negative for inflammatory bowel disease or colon cancer. On examination, she is vitally stable. Her abdominal examination is normal. Stool analysis is negative for occult blood, parasites, or ova. Which of the following is the most appropriate diagnostic test to perform?