A 60-year-old male presented to the ER complaining of abdominal distension with discomfort and obstipation for 2 days. On examination, his abdomen was diffusely distended with mild tenderness, tympanic on percussion and sluggish bowel sounds. On PR, the rectum was empty. Vital signs were stable. There was marked colonic dilatation on X-ray. Rigid sigmoidoscopy revealed gradual narrowing of the lumen at 20 cm with a normal mucosa. What is the most likely diagnosis?