**Core Concept**
The condition described is a classic presentation of herpes zoster (shingles), a viral infection caused by the reactivation of latent varicella-zoster virus (VZV) in the nervous system. This reactivation leads to inflammation and damage to the sensory nerves, resulting in characteristic skin lesions and pain.

**Why the Correct Answer is Right**
The varicella-zoster virus typically resides in the dorsal root ganglion (DRG), a cluster of nerve cell bodies located near the spinal nerve roots. When VZV reactivates, it travels down the sensory nerve axons to the skin, causing the characteristic rash and pain. The DRG serves as the site of latency for VZV, allowing it to remain dormant until reactivation occurs, often triggered by factors such as aging, stress, or immunosuppression.

**Why Each Wrong Option is Incorrect**
**Option A:** The brainstem is not directly involved in the pathogenesis of herpes zoster, which primarily affects the peripheral nervous system.
**Option C:** While the sensory nerve axon is involved in the transmission of the virus to the skin, it is not the primary site of latency for VZV.
**Option D:** The sensory nerve root is a bundle of nerve fibers that emerges from the spinal cord, but it is not the specific location where the varicella-zoster virus resides during latency.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the characteristic distribution of herpes zoster lesions, which typically follows the dermatomal pattern of the affected nerve roots. In this case, the medial aspect of the right arm, extending from the medial epicondyle to the wrist, suggests involvement of the C8-T1 dermatomes.

**✓ Correct Answer: B. Dorsal root ganglion**