A 35-year-old female presents to the orthopedic clinic 6 weeks after sustaining a fall on her outstretched right hand. She reports persistent wrist pain and weakness in grip strength. Radiographs show a fracture of the distal third of the radius with dorsal angulation, and the distal radioulnar joint (DRUJ) appears widened. Clinical examination reveals tenderness over the distal radioulnar joint and loss of pronation. What is the most likely diagnosis and the primary reason for her functional deficit?