The lumbar spine xrays sound normal except for the sclerosis, which is an area of scarring in the bone.
Often sclerosis is due to degenerative joint disease, but it can be due to back injuries, or instability in the spine due to things such as fractures, infections, or rarely things like tumors or problems in the way your body utilizes calcium or other minerals. The MRI is much better than a plain xray at examining the soft tissues in the spine such as the ligaments, disks and nerves. The bone scan uses special substances called "tracers" to locate areas of inflammation, which usually correlate with things such as infections or tumors.
Your doctor is probably just being on the safe side to make sure there is no other unusual cause for your back pain and this area of bone scarring.
Best wishes to you.