A 48-year-old woman has a lower back pain and hypoesthesia in the left S1 dermatomal distribution (left calf and lateral left foot). What is the most likely cause?

Thoracic, lumbar, and sacral nerves exit off the spinal canal immediately below the pedicle of the corresponding numbered veebra. The left S1 root, for example, passes immediately dorsal to the L5-S1 disc where it can be susceptible to compression by a herniated nucleus pulposus. The root then swings laterally to exit immediately caudal to the left L5 pedicle. For a correlation between level of disc herniation and the root affected, see the table below. Level of Herniation Root Affected L1-L2 L2 L2-L3 L3 L3-L4 L4 L4-L5 L5 L5-S1 S1