A 40-year-old male developed right thoracic back pain 2 years ago, right lower limb weakness 1.5 years ago, diminished pain and temperature sensation in the left lower limb, bilateral lower limb weakness over the past 6 months, difficulty climbing stairs, urinary retention and constipation for 3 months, and inability to walk for 1 month. Examination: Normal upper limbs, muscle strength grade 2 in lower limbs, increased muscle tone (hypertonia), hyperactive knee reflex, positive bilateral Babinski signs, decreased pain, temperature, and tuning fork sensation below T7-T8. The most likely diagnosis is: