A 68-year-old man presents to the emergency department with acute-onset severe abdominal pain and vomiting for 6 hours. He has a history of previous abdominal surgery 15 years ago. On examination, he is in distress, with a distended abdomen and hyperactive bowel sounds. Plain abdominal radiograph (supine and erect views) shows multiple dilated small bowel loops with a "step-ladder" appearance, and air-fluid levels at different heights on the erect film. No free air is visible under the diaphragm. What is the most likely diagnosis?