A dermatomal distribution is most consistent with disease affecting which structure?

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

Answer: (C)

A dermatomal distribution reflects the skin areas supplied by a single spinal nerve root. Each dermatome corresponds to one spinal level, so when a root is affected—such as by radiculopathy or reactivation of varicella zoster in the dorsal root ganglion—the sensory symptoms or rash appear in a narrow belt that encircles a limb or trunk. This segmental, belt-like pattern is the hallmark of spinal nerve root involvement rather than the pattern expected from brain cortex lesions (which don’t respect skin segments) or peripheral nerve disease (which follows a nerve’s own distribution that isn’t confined to a single dermatome). Involvement of cranial nerves affects specific nerve territories, not a spinal dermatome, so the spinal nerve roots best fit the dermatomal pattern.