A 50-year-old with L4-L5 disc prolapse develops weakness of knee extension, reduced knee jerk, and sensory loss over the medial leg and dorsum of foot. Which nerve root is most likely compressed, and why does L4 radiculopathy produce these specific findings?

• A L5 nerve root compression; L5 weakness affects tibialis anterior (foot drop) and supplies dorsolateral foot and great toe
• B L4 nerve root compression; L4 supplies quadriceps (knee extension) via femoral nerve and skin over medial leg and dorsal foot, with the knee jerk testing L3-L4 arc ✓
• C S1 nerve root compression; S1 weakness affects plantar flexion and ankle jerk with lateral foot numbness
• D L3 nerve root compression; L3 supplies hip flexion and adduction with sensation over anterior thigh

Explanation

L4 nerve root carries motor fibers to the quadriceps femoris (knee extension) via the femoral nerve (L2, L3, L4), and sensory fibers over the medial aspect of the leg and ankle (via the saphenous branch). The knee jerk (patellar reflex) tests the L3-L4 arc. An L4-L5 disc prolapse typically compresses the L4 root (posterolateral direction affects the traversing root at that level) causing these findings. L5 compression produces foot drop (weak tibialis anterior) and sensory loss on dorsal foot and great toe with no reflex change (L5 has no accessible deep tendon reflex). S1 affects ankle jerk, plantar flexion, and lateral foot sensation.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.