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 system is causing kai's pain and numbness?

the peripheral nervous system
the central nervous system
the endocrine system
the cirulatory system

"The peripheral Nervous system"

Your nervous system is split into two halves the CNS (Central Nervous) and PINS (peripheral). As the name suggest Central contains the organs in the center: the brain and spinal cord. Peripheral contains the nerves that protrude from the CNS. Remember you can get a lot of information by paying attention to the wording.

The peripheral nervous system.  peripheral nervous system controls outside the brain and spinal chord. endocrine and circulatory don't exactly control the nerves but work with the nervous system. the central nervous system controls the brain and the spinal chord which aren't in relation to the legs.