20. A 78-year-old man presents to the emergency department w…
20. A 78-year-old man presents to the emergency department with acute onset of weakness in the lower extremities and difficulty walking. On examination he has normal strength in the upper extremities. Lower extremities are flaccid with absent reflexes. Sensory examination shows decreased temperature sensation to cold and decreased pinprick below the umbilicus. Vibration and joint position sense are preserved. What is the most likely mechanism causing his symptoms?