A 72-year-old woman with no prior abdominal surgery presents with progressive abdominal distension, constipation, and colicky pain for 5 days. On examination, she is afebrile but tachycardic (HR 110/min), abdomen is massively distended with absent bowel sounds. Plain abdominal X-ray shows a hugely dilated colon (diameter 12 cm) with a transition zone at the rectosigmoid junction and no small bowel dilatation. What is the most appropriate next step in management?