What is the nerve root injury that causes drop foot?

Deep peroneal: Isolated drop foot is due to deep peroneal (aka fibular) nerve injury on lateral side of the knee or below; if there is also difficulty turning the foot in, the common peroneal nerve may be involved (lateral side of the knee or a little above).+loss of leg/foot sensation also.It may also be due to L5 nerve root injury in the lower back from degenerative disc disease/disc bulging at the l4-5 level.