History: young adult male with back pain. 

This is a case of a pars interartcularis defect with anterolisthesis, also called spondylolysis with spondylolisthesis. These injuries are thought to be due to chronic repetitive stress at the pars interarticularis, and most commonly occur at the L5-S1 level. It is best demonstrated on oblique radiographs of the lumbar spine where discontinuity of the neck of the Scotty Dog is demonstrated, which you can read about here.

On the frontal view, spondylolysis with spondylolisthesis can be diagnosed by the inverted Napolean hat sign, which you can read about here.