A 33-year-old male patient returns from a 6-year tour of navy duty where he was stationed in Thailand. The patient presents with impairment of proprioception and vibratory sensation along with loss of deep tendon reflexes. When the patient walks or stands, he maintains a wide-base gait due to his sensory ataxia. When asked to stand feet together with his eyes closed, he displays Rombergs sign (unsteady, swaying posture). He is diagnosed with tabes dorsalis due to tertiary neurosyphilis. What is the most likely location of this lesion?