A 58-year-old man presents with a 3-month history of progressive left-sided leg pain radiating to the lateral foot and dorsum, with recent onset of foot drop and difficulty walking. On examination, ankle dorsiflexion is weak (grade 3/5). A lateral lumbar spine X-ray is obtained. The intervertebral disc space marked **B** (L4-L5) shows narrowing with a vacuum phenomenon visible. Based on the radiologic findings and clinical presentation, which of the following best explains the nerve root affected and the mechanism of compression?

Explanation

## Why L5 nerve root compressed by posterolateral L4-L5 disc herniation; foot drop results from weak ankle dorsiflexion is right The L4-L5 intervertebral disc space marked **B** shows narrowing with vacuum phenomenon, indicating degenerative disc disease. When a disc herniates posterolaterally at L4-L5, it compresses the **exiting nerve root BELOW** — the L5 root — because the L4 root has already exited through the L4-L5 neural foramen above. L5 radiculopathy presents with lateral leg pain, dorsal foot pain, and weakness of ankle dorsiflexion (tibialis anterior), causing a characteristic steppage gait and foot drop. This patient's clinical presentation (lateral foot pain, foot drop, weak dorsiflexion) is pathognomonic for L5 radiculopathy. [Gray's Anatomy 42e Ch 23; Apley 10e] ## Why each distractor is wrong - **L4 nerve root compressed by posterolateral L4-L5 disc herniation; foot drop results from weak ankle plantarflexion**: The L4 root exits at the L4-L5 foramen above the disc; it is not compressed by L4-L5 disc herniation. L4 radiculopathy causes medial calf pain and quadriceps weakness, not foot drop from dorsiflexion weakness. - **L5 nerve root compressed by central L4-L5 disc herniation; foot drop results from weak ankle plantarflexion**: While L5 is correctly identified, central disc herniation typically causes cauda equina syndrome (bilateral symptoms, saddle anesthesia, urinary retention), not isolated unilateral L5 radiculopathy. Posterolateral herniation is the classic mechanism for single-root compression. Additionally, L5 radiculopathy causes dorsiflexion weakness, not plantarflexion weakness. - **S1 nerve root compressed by posterolateral L4-L5 disc herniation; foot drop results from weak ankle dorsiflexion**: S1 radiculopathy causes posterior leg pain, lateral foot pain, and ankle plantarflexion weakness (gastrocnemius). The S1 root exits at L5-S1, not L4-L5. Foot drop from dorsiflexion weakness is L5, not S1. **High-Yield:** L4-L5 disc herniation → L5 root (not L4); L5 radiculopathy → lateral leg + dorsal foot pain + ankle dorsiflexion weakness + steppage gait/foot drop. [Gray's Anatomy 42e Ch 23; Apley 10e]