A 2-year-old is brought to the emergency department by his mother for intermittent abdominal pain. She states the pain seems to be relieved by flexing his knees to his chest. The mother states that she noticed this morning that he passed stool mixed with blood and mucus. On exam, you note abdominal tenderness. An abdominal ultrasound is performed, and the results are shown in Figure A. What is the most common underlying cause of this patient's condition?