A 45-year-old man presents with low back pain radiating to the right leg in an L5 dermatomal pattern (big toe and dorsum of foot numbness), weakness of extensor hallucis longus, and a normal ankle jerk. MRI shows a posterolateral disc herniation at L4-L5. Which nerve root is compressed?

• A S1 nerve root
• B L4 nerve root
• C L5 nerve root ✓
• D L3 nerve root

Explanation

At L4-L5, a posterolateral disc herniation compresses the traversing L5 root (not the exiting L4 root). L5 supplies extensor hallucis longus (EHL) — the classic weak muscle — and sensation over the dorsum of the foot and big toe. The ankle jerk (S1) is normal, distinguishing L5 from S1 compression. S1 compression at L5-S1 causes weak plantar flexion, absent ankle jerk, and lateral foot numbness.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.