Meckel diverticulum

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?