Which nerve root is most commonly affected by a posterolateral disc herniation at the L4-L5 level?

Explanation
In the lumbar spine, a posterolateral disc herniation typically affects the traversing nerve root. Therefore, an L4-L5 herniation compresses the traversing L5 root, while an L5-S1 herniation affects the S1 root. The exiting L4 root is usually spared unless it is a far lateral (foraminal) herniation.