A hоrse shоws аn increаsing level оf аbdominal pain during a 12-hour period. On physical examination you detect a heart rate of 60 beats/min, dark pink mucosae, decreased intestinal sounds, and no abnormalities on rectal examination. Passage of a nasogastric tube produces 20 liters of net gastric reflux. The horse appears much more comfortable after removal of the gastric fluid. On abdominocentesis the peritoneal fluid has a normal white blood cell count, a slightly elevated protein content, and is straw-colored. The most likely cause of these findings is: