## Dermatome Distribution of the Upper Limb **Key Point:** The C6 dermatome covers the lateral forearm, thumb, and index finger — making it the best explanation for a band-like sensory loss across the lateral arm AND forearm. ### Anatomical Basis The dermatomes of the cervical spine follow a segmental pattern in the upper limb: - **C5**: Lateral shoulder and lateral arm (deltoid patch area); does NOT extend significantly into the forearm - **C6**: Lateral forearm, thumb, and index finger — the classic "lateral arm and forearm" band - **C7**: Middle finger, dorsal hand, posterior forearm - **C8**: Medial forearm, ring and little fingers, medial hand ### Why C6 is the Answer The stem describes sensory loss in a "band-like distribution across the lateral arm and forearm" with preserved medial arm sensation. This pattern is characteristic of C6 dermatome involvement. C5 is limited primarily to the lateral shoulder/upper arm (deltoid region) and does not extend into the forearm. C6, by contrast, sweeps from the lateral arm down through the lateral forearm to the thumb and index finger — precisely the territory described. **Clinical Pearl:** A useful mnemonic — C6 = "6 o'clock on the lateral side" — it runs the full lateral column of the forearm. Preservation of medial arm sensation (C8/T1) confirms the lesion is at C6, not a lower root. **High-Yield (per Snell's Clinical Anatomy / Gray's Anatomy):** The "lateral to medial" progression in the upper limb: C5 (lateral shoulder/arm), C6 (lateral forearm to thumb), C7 (middle finger/dorsum), C8 (medial forearm/little finger). A stab wound to the posterior shoulder injuring the C6 root or posterior cord at that level would produce exactly this lateral arm-and-forearm band of sensory loss. 
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