A 62-year-old man from rural Maharashtra presents with acute onset colicky abdominal pain, abdominal distension, and absolute constipation for 18 hours. On examination, he has visible peristaltic waves and a palpable mass in the left lower abdomen. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels and a transition zone in the left colon. CT abdomen with IV contrast shows a shouldering sign at the splenic flexure with shouldering of the colon wall. What is the most appropriate next step in management?