A 62-year-old man with a history of abdominal surgery 15 years ago presents with acute onset colicky abdominal pain, abdominal distension, and bilious vomiting for 8 hours. On examination, he is dehydrated with visible peristaltic waves. Plain abdominal X-ray shows dilated small bowel loops (diameter 4 cm) with multiple air-fluid levels in a "staircase" pattern, and minimal colonic gas. CT abdomen with IV contrast reveals transition zone with abrupt change in caliber at the distal ileum, with no evidence of mass, perforation, or ischemia. What is the most likely diagnosis?