A 38-year-old female from Mumbai slipped on a wet floor and twisted her right ankle inward. She presents with moderate pain and swelling localized to the lateral ankle. On examination, the ankle is stable with no gross deformity. Plain radiographs show a fracture of the fibula 6 cm proximal to the ankle joint with an intact medial malleolus and no posterior malleolar involvement. There is widening of the medial ankle space on the mortise view. What is the most appropriate diagnosis?