A 5-year-old girl is brought to the pediatrician for evaluation of
abdominal pain and bloody stools. The symptoms began three-weeks ago. The abdominal pain is intermittent and usually self-resolves within an hour. Past medical history is noncontributory. She has been meeting all
developmental milestones. Physical examination reveals a nontender and nondistended abdomen with normal
bowel sounds. Laboratory testing reveals the following:
| Laboratory value | Result |
| Hemoglobin | 15.2 g/dL |
| Platelet count | 260,000/mm3 |
| Prothrombin time | 14 seconds |
| Partial thromboplastin time | 32 seconds |
A radiolabeled technetium-99 scan is performed and reveals increased uptake in a structure located in the right lower abdominal quadrant. The walls of this structure will most likely consist of which of the following?
