EMPLOYEE REPORTS SLIPPING ON A STEP WHILE EXITING A BUILDING BETWEEN SILOS & A-FRAME. EE TWISTED HIS LEFT ANKLE. EE WAS LEAVING PREHEATING AREA THROUGH UNCOMMONLY USED DOORWAY, STEPPED ONTO UNFASTENED STEP, STEP SHIFTED. EE STATES HE HAD A CRACKING NOISE COME FROM HIS ANKLE & REPORTS STIFFNESS, SWELLING & SHARP PAIN.