After injuring his knee kai began having pain and numbness in his lower leg. the doctor said the injury damaged nerves that run all the way down his leg
damage to which systems is causing kai pain and numbness

Damage to the nervous system is causing Kai to have pain and numbness.  

Kai injured his knee. We know that the posterior surface of the knee has the quadrangular fossa which is guarded by the tendons of hamstrings anteriorly and by the two heads of gastrognemus muscle below. There are several nerves running down the leg through this region.

Any trauma to this fossa can cause these nerves to get damaged. Thus it can lead to numbness of lower limb. Proper physiotherapy is required to recover as soon as possible. Else that can even lead to paralysis of lower limb.  

did you fail the test? xd