Which of the following events occurs fourth in sequence after a fracture of a typical long bone?

The answer is D. Following fracture of a long bone provisional callus is formed by the proliferation of periosteal inner layer (E) which forms a cuff of bone trabeculae which produce the external callus; medullary cavity reaction (A) results in organization of the fibrin clot with production of woven bone in the marrow spaces; cortical reaction (B) results in increased osteoclastic resorption. The external callus unites the fragments externally, but not the bone ends which are joined by the fibrin clot and debris which in turn are ossified by osteogenic cells from the medullary cavity and periosteal callus (D). The final steps is remodelling of the bone (C) with formation of lamellar bone and resorption of the external callus, and eventually medullary callus.