A 50-year-old man presents to the emergency in a wheelchair complaining of weakness that has developed over the past 2 days. He first noticed that he had generalized fatigue and felt like he was having a hard time moving his feet. Over the past 24 hours, the weakness has progressed to the point that he can barely stand with assistance. He is beginning to feel that it is difficult to lift his arms. He also complains of a sharp pain in his shoulders and along his spine. Both his hands and feet are tingling. On physical examination, his initial blood pressure is 140/80 mmHg. On repeat 1 hour later, it is 90/50 mmHg. His hea rate is 106 bpm, respiratory rate is 25 breaths/min, temperature is 37.0degC, and SaO2 is 96% on room air. Deep tendon reflexes are absent at the knee, ankle, and wrist. The brachioradialis reflex is 1+. Strength throughout the lower extremities is diminished as the patient is unable to lift either leg against gravity. What is the appropriate diagnosis?