A 58-year-old woman with no prior abdominal surgery presents with acute abdominal pain, vomiting, and constipation for 12 hours. On examination, she is afebrile with a soft, distended abdomen. Plain radiograph shows a massively dilated colon with a 'coffee bean' appearance and a transition point at the sigmoid region. CT abdomen confirms a twisted sigmoid colon with a whirled mesentery sign. What is the most likely diagnosis, and what is the next step in management?