A 38-year-old man presents with acute lower back pain radiating to the lateral aspect of the right leg and dorsum of the foot. On examination, there is weakness of foot dorsiflexion and sensory loss over the dorsum of the foot and lateral leg. Which is the most common site of nerve root compression causing this clinical presentation?

## Clinical Presentation Analysis **Key Point:** The combination of foot dorsiflexion weakness, sensory loss over the dorsum of the foot and lateral leg, and radicular pain to the lateral leg is pathognomonic for L5 nerve root compression. ### Dermatome and Myotome Correlation | Nerve Root | Sensory Distribution (Dermatome) | Motor Innervation (Myotome) | Common Reflex | |---|---|---|---| | **L5** | Dorsum of foot, lateral leg, lateral foot | Tibialis anterior (foot dorsiflexion), extensor hallucis longus, peroneal muscles | Medial hamstring reflex | | L4 | Medial leg, medial foot, anterior thigh | Quadriceps, tibialis anterior | Patellar reflex | | S1 | Lateral foot, sole, posterior leg | Gastrocnemius, soleus (plantarflexion), gluteus maximus | Achilles reflex | | L3 | Anterior and medial thigh | Quadriceps, adductors | Patellar reflex | ### Why L4–L5 Disc Herniation Is Most Common 1. **Anatomical vulnerability:** The L4–L5 disc is the most mobile segment of the lumbar spine and bears the greatest load during flexion and rotation. 2. **Epidemiological data:** L4–L5 and L5–S1 are the two most common sites of disc herniation, but L4–L5 accounts for approximately 45–50% of all lumbar disc herniations [cite:Gray's Anatomy 41e]. 3. **Clinical correlation:** When L4–L5 disc herniates posterolaterally, it compresses the exiting L5 nerve root (which exits above the L5 vertebra), producing the classic L5 radiculopathy pattern. **High-Yield:** Remember that a disc hernia at L4–L5 compresses the L5 root (not L4), because nerve roots exit above their corresponding vertebra in the lumbar spine. **Clinical Pearl:** Foot dorsiflexion weakness is the most sensitive motor sign for L5 radiculopathy and should always trigger imaging of the L4–L5 disc space.