Nerve Root Affected: L5-S1 (Option C)
The patient's presentation of sensory loss in the left calf and lateral aspect of the left foot indicates L5-S1 nerve root involvement, specifically affecting the L5 nerve root. 1
Neuroanatomical Correlation
The L5-S1 disc lesion typically compresses the L5 nerve root as it exits the neural foramen, producing characteristic sensory deficits in specific distributions 1:
- L5 nerve root compression causes sensory loss in the dorsal aspect of the foot, including the big toe 1
- The lateral aspect of the foot and calf falls within the L5 dermatome distribution 1
- This differs from S1 nerve root compression, which produces lateral foot and plantar flexion deficits 2
Why Other Options Are Incorrect
L1-L2 (Option A): This nerve root level affects the lateral thigh region, not the calf or foot 2
L3-L4 (Option B):
- L4 nerve root affects the medial lower leg and medial thigh, not the lateral calf or lateral foot 3, 2
- L4 radiculopathy typically presents with diminished patellar reflex and medial leg sensory changes 3
S1-S2 (Option D):
- S1 nerve root primarily affects plantar flexion strength and the Achilles reflex 2
- While S1 can affect the lateral foot, it does not typically cause calf sensory loss in the distribution described 1
Clinical Confirmation
To confirm L5 nerve root involvement, assess for:
- Motor deficits: Foot dorsiflexion weakness and great toe extension weakness (L5-mediated movements) 1
- Reflex changes: Ankle jerk reflex may be affected rather than knee jerk reflex 1
- Sensory pattern: Dorsal foot and big toe sensory loss is the hallmark finding 1