A 45-year-old construction worker presents with low back pain radiating to the lateral calf and dorsum of the foot, with weakness of dorsiflexion of the ankle and loss of sensation over the first web space. The disc herniation responsible is compressing which nerve root at which level?

• A L3 nerve root at the L3–L4 disc
• B L5 nerve root at the L4–L5 disc ✓
• C L4 nerve root at the L4–L5 disc
• D S1 nerve root at the L5–S1 disc

Explanation

An L4–L5 postero-lateral disc herniation compresses the L5 nerve root (which exits below L5). L5 root compression produces weakness of ankle dorsiflexion (tibialis anterior) and big-toe extension (extensor hallucis longus), with sensory loss over the lateral leg, dorsum of the foot, and the first web space. The knee jerk (L4) and ankle jerk (S1) are typically preserved. S1 root compression produces calf and plantar sensory changes, weak plantar flexion, and absent ankle jerk.

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

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Written and medically reviewed by the StethoPrep medical team.