A 3-year-old girl is seen in the emergency depament with acute abdominal pain. She has a 5-day history of vomiting and abdominal distension. She has not passed stool during this time, and during the past day, has been vomiting bilious material. On physical examination, she is lethargic, with a firm and tender abdomen, and peritoneal signs are present. She is immediately referred for laparotomy for suspected diagnosis of intussusception vs. volvulus. At surgery, approximately 20 cm of small intestine is found to be markedly distended, and is resected. The section contains a tightly knotted ball of nematodes that are about 15 to 35 cm in length. The worms have tapered ends without hooks. Pa of the life cycle of this patient's parasite is a filarial stage in which larvae, hatched in the duodenum, penetrate the wall of the small intestine. The passage of migrating larvae most commonly produces symptomatic disease in which of the following organs?