A 72-year-old woman from Delhi presents with acute onset severe left lower abdominal pain, abdominal distension, and absolute constipation for 3 days. She has a history of chronic constipation and neurological disease (Parkinson's). On examination, she is afebrile, blood pressure 130/80 mmHg, and has mild abdominal tenderness with palpable abdominal mass in the left lower quadrant. Plain abdominal radiograph shows a single massively dilated loop of colon in the left abdomen with a characteristic "bird's beak" appearance at the rectosigmoid junction. What is the most likely radiographic diagnosis?