A 58-year-old woman with no prior abdominal surgery presents with acute severe abdominal pain, nausea, and vomiting. On examination, she is in distress with a palpable abdominal mass in the right lower quadrant. Plain abdominal X-ray shows dilated small bowel loops and a large-diameter colon (8 cm) with a "bird's beak" appearance at the hepatic flexure on CT. There is no free perforation. What is the most likely diagnosis?