Question

An otherwise healthy 34-year-old man presents to the emergency department with a 4-day history of progressive weakness in his legs. He reports numbness that started in his toes and has now ascended to his umbilicus, accompanied by a sensation of a "tight belt" around his mid-abdomen. Earlier today, he was unable to void urine for $12$ hours despite feeling a strong urge. Three weeks ago, he experienced a self-resolving episode of crampy abdominal pain and bloody diarrhea. His temperature is $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), blood pressure is $128/82\text{ mmHg}$, and pulse is $88/\text{min}$. Physical examination reveals $3/5$ strength in the bilateral lower extremities and $5/5$ in the upper extremities. Deep tendon reflexes are $3+$ at the patellae and Achilles bilaterally. Sustained ankle clonus is present bilaterally, and plantar responses are upgoing. Pain and temperature sensation are absent below the level of the umbilicus. Sensation and reflexes in the upper extremities are normal. There is no spinal tenderness to percussion. Laboratory studies show a leukocyte count of $7,500/\mu\text{L}$, normal erythrocyte indices, and a serum vitamin $B_{12}$ level of $245\text{ pg/mL}$ (normal: $200$ to $900\text{ pg/mL}$). Which of the following is the most likely diagnosis?