Dermatomal distribution is best explained by involvement of which anatomical structure?

• A. Cranial nerves
• B. Spinal nerve roots
• C. Brain
• D. Peripheral nerves

Answer: (B)

Dermatomes map the skin areas that receive sensory input from a single spinal nerve root. When a nerve root is affected—such as in radiculopathy or shingles—the resulting sensory changes (pain, numbness, or paresthesia) follow a belt-like pattern that corresponds to that specific root’s dermatome. Brain lesions disrupt sensation in a more generalized or cortical pattern and do not respect dermatomal boundaries. Cranial nerves explain sensory findings in the face and head regions served by those nerves, not the typical body dermatomes. Peripheral nerves produce sensory territories tied to that nerve’s distribution, which can cross dermatomal lines and don’t align with the organized dorsal root map. Therefore, involvement of spinal nerve roots best explains dermatomal distribution.