A 69-year-old man comes to the physician because of the gradual onset of pain, tingling and hyperesthesia in the medial aspect of his right arm. Shortly thereafter, he develops erythema and then an outbreak of vesicles on the medial aspect of his right arm extending from his medial epicondyle to the wrist. After several days. the lesions crust over and eventually resolve. However, he is left with a residual “burning” pain in the same distribution as the lesions with occasional episodes of sharp.1O/1O pain that is provoked by touch. He uses ice packs to suppress the pain at night but it has persisted despite this. The infectious agent responsible for this condition resides in which part of the neuraxis?