A 45-year-old man presents with severe pain and swelling over the forearm following a high-energy motor vehicle accident. Plain radiographs show a fracture of the distal radius shaft with widening of the space marked **C** (radioulnar joint and interosseous space) and loss of the normal relationship between the radius and ulna at the wrist. Which of the following fracture patterns is most likely responsible for this imaging finding and mandates urgent operative intervention?