A 62-year-old man presents with acute onset colicky abdominal pain, bilious vomiting, and abdominal distension. He has a history of open appendectomy 15 years ago. CT abdomen with IV contrast shows the findings marked **A** in the diagram — dilated small bowel loops with air-fluid levels and a discrete transition point. The proximal bowel is dilated (>3 cm) with the feculent material sign, while the distal bowel is collapsed. His vital signs are stable, and there is no peritonitis. Which of the following is the most likely etiology of his small bowel obstruction?