Ee was prep to clean the A/C of the control tower with the water trk but while exiting the truck ee stepped on a rock and sprained left knee. The ee sought medical attn on 7/24 and was diag with a sprain and released w/o rest. MRI results were obtained which revealed a hairline fracture of the femoral condyle which made this as a med reportable injury as of 8/7/17.