Review Question - QID 210656

A 44-year-old man presents to his primary care physician due to a shock-like pain in his left leg. He describes the pain as sharp and lasting a few minutes at a time. He has noticed being "clumsy" when walking in a dark room. Approximately 2 weeks ago, he was helping his daughter move and this required him to lift heavy boxes. He denies any trauma to the back or lower back pain. Medical history is significant for hypertension, hyperlipidemia, and type 2 diabetes mellitus. He is currently sexually active and is in a monogamous relationship with his wife. Approximately 15 years ago, he noted a painless genital lesion. On physical exam, there is a miotic pupil that does not constrict with light but constricts with convergence and accommodation. Strength, reflex, and sensory exam of the lower extremity is unremarkable. The patient has a positive Romberg test. Which of the following is most likely the cause of this patient's symptoms?