Spondylolysis is a condition in which there is a defect in the pars interarticularis of the vertebra. The pars interarticularis is a small segment of bone from the vertebral arch joining the facet joints of the spine. An MRI scan can evaluate changes in the par interarticularis to diagnose active spondylolysis or inactive spondylolysis. Active spondylolysis indicates that there is movement at the pars defect that is likely to cause low back pain, while inactive spondylolysis indicates no movement at the defect and low back pain is from another source. Patients are often advised to limit rigorous activities like sports to avoid causing advancement of the spondylolysis that may lead to the development of spondylolisthesis.