• a trauma, surgery, injection, or compression site
• a specific motor deficit
• a sensory-loss distribution
• a characteristic gait or posture

Topic map

1. Lumbosacral plexus and root overview
   • Lumbar plexus: femoral, obturator, lateral femoral cutaneous
   • Sacral plexus: sciatic, superior/inferior gluteal, pudendal
   • Root lesions versus named peripheral-nerve lesions
2. Femoral nerve
   • Hip flexion and knee extension weakness
   • Reduced patellar reflex
   • Sensory loss over anterior thigh and medial leg
   • Common settings: pelvic/retroperitoneal bleeding, pelvic surgery
3. Obturator nerve
   • Weak thigh adduction
   • Medial thigh sensory deficit
   • Pelvic surgery or obturator canal injury
4. Lateral femoral cutaneous nerve
   • Meralgia paresthetica
   • Purely sensory burning/numbness of anterolateral thigh
   • Compression under inguinal ligament, obesity, pregnancy, tight clothing
5. Sciatic nerve
   • Posterior hip dislocation and gluteal injection injury
   • Hamstring weakness plus deficits below the knee
   • Tibial and common fibular division findings
6. Common fibular (peroneal) nerve
   • Fibular neck injury
   • Foot drop, loss of dorsiflexion and eversion
   • Steppage gait
   • Sensory loss over dorsum of foot and lateral leg
7. Tibial nerve
   • Tarsal tunnel syndrome or popliteal injury
   • Impaired plantarflexion and inversion
   • Sensory loss on sole of foot
   • Difficulty standing on tiptoes
8. Superior and inferior gluteal nerves
   • Superior gluteal: Trendelenburg sign/gait, failed hip abduction
   • Inferior gluteal: difficulty rising from a chair or climbing stairs due to weak hip extension
9. Root-level patterns that mimic peripheral injuries
   • L4/L5/S1 radiculopathies
   • Disc herniation patterns, reflex changes, and differentiating root versus nerve lesions
10. NBME/UWorld approach

• Localize by first identifying the failed action
• Confirm using the sensory territory
• Use the injury mechanism to distinguish close answer choices

High-level test-taking framework

1. What movement is weak?
   Example: inability to dorsiflex means deep fibular nerve or L5 root.
2. Which movements are preserved?
   Foot drop with preserved inversion points to common fibular injury. Foot drop with weak inversion too suggests L5 radiculopathy, because inversion is supplied by tibialis posterior via the tibial nerve but has major L5 input.
3. Where is sensory loss?
   • First dorsal web space: deep fibular nerve
   • Dorsum of foot: superficial/common fibular
   • Sole: tibial
   • Medial leg: saphenous branch of femoral
   • Anterolateral thigh only: lateral femoral cutaneous
4. What was injured or compressed?
   • Fibular neck: common fibular
   • Posterior hip dislocation: sciatic
   • Gluteal injection: sciatic
   • Inguinal ligament: lateral femoral cutaneous
   • Pelvic hematoma: femoral
   • Piriformis region: sciatic

1. Complete nerve-by-nerve table
2. Foot drop and root-vs-nerve localization
3. Femoral, obturator, and meralgia paresthetica
4. Sciatic, fibular, tibial, and gluteal nerves
5. UWorld/NBME-style practice questions