A 72-year-old man with a long history of chronic constipation presents with acute onset of abdominal pain, distension, and inability to pass stool or flatus for 2 days. On examination, he is afebrile, BP 130/80, with a markedly distended abdomen. Abdominal X-ray shows a hugely dilated sigmoid colon with a characteristic "coffee-bean" or "omega" configuration. The transition point is at the rectosigmoid junction. What is the most appropriate next step in management?