Final answer:
The interosseous membrane is the key structure that prevents distal radioulnar joint reduction in Galeazzi fractures, with the triangular fibrocartilage complex also playing a role.
Step-by-step explanation:
The tendon that prevents distal radioulnar joint (DRUJ) reduction in Galeazzi fractures is the interosseous membrane and, to a lesser extent, the triangular fibrocartilage complex (TFCC). A Galeazzi fracture is a fracture of the radial shaft with associated dislocation of the DRUJ. Proper treatment typically involves anatomical reduction of the radius and surgical fixation to allow the DRUJ to reduce indirectly. If the interosseous membrane is intact, it can prevent the reduction of the DRUJ after the radial fracture is repaired because it maintains the proximal migration of the radius relative to the ulna.