A 58-year-old man presents with a 3-month history of progressive left leg pain radiating to the dorsum of the foot and lateral calf, worse with walking and standing. MRI of the lumbar spine is performed. The structure marked **C** in the diagram shows a focal disc protrusion. Which of the following nerve roots is most likely to be compressed by pathology at this level?

Explanation

## Why L5 nerve root (traversing L4-L5 foramen) is right In the lumbar spine, the nerve root that exits at a given level does so ABOVE the corresponding disc. At the L4-L5 level (marked **C**), the L4 root exits above the disc at the L4-L5 foramen, while the L5 root traverses the L4-L5 foramen as it descends to exit at the L5-S1 foramen. Therefore, a disc herniation at L4-L5 typically compresses the L5 nerve root (the traversing root), not the L4 root. The patient's clinical presentation of lateral leg pain, dorsum of foot pain, and L5 dermatome distribution confirms L5 radiculopathy. (Sutton Radiology; Maheshwari Orthopedics 10e) ## Why each distractor is wrong - **L4 nerve root (exiting at L4-L5 foramen)**: The L4 root exits ABOVE the L4-L5 disc at the L4-L5 foramen and is therefore not typically compressed by L4-L5 disc herniation. The exiting root is protected; the traversing root below is at risk. - **L3 nerve root (exiting at L3-L4 foramen)**: The L3 root exits at the L3-L4 level, which is superior to the L4-L5 disc. Pathology at L4-L5 does not affect L3. - **S1 nerve root (traversing L5-S1 foramen)**: While S1 can be compressed by L5-S1 disc herniation, the question specifically refers to the L4-L5 level (**C**). S1 radiculopathy would present with posterior leg, sole, and Achilles reflex changes—not the dorsum of foot pain described. **High-Yield:** In lumbar spine, the nerve root that EXITS at a level does so ABOVE the disc; the nerve root that TRAVERSES a foramen below the disc is at risk from herniation at that level (e.g., L4-L5 herniation → L5 radiculopathy). [cite: Sutton Radiology; Maheshwari Orthopedics 10e]