A 58-year-old woman presents with acute abdominal pain, constipation, and abdominal distension. She has no prior abdominal surgery. On examination, the abdomen is markedly distended and tympanic. Plain abdominal radiograph shows a massively dilated loop of colon with a characteristic "coffee bean" appearance and a transition point at the sigmoid-descending colon junction. The small bowel is not dilated. What is the most likely diagnosis, and what is the next step in management?