A 24-year-old male developed insidious left-sided electric shock-like chest pain 7 months ago, worsening at night. Progressive left lower limb weakness appeared 4 months ago, followed by right lower limb weakness and urinary retention 1 month later. Examination shows normal upper limbs, left lower limb muscle strength 3/5, right 4/5, increased muscle tone (hypertonia), hyperactive tendon reflexes, positive bilateral Babinski signs, and hypoesthesia below the nipples. The most likely diagnosis is: