A young otherwise-healthy man is learning to surf for the first time. After his first lesson, he begins to develop progressively worsening low back pain, followed by lower extremity paresthesias. On arrival to urgent care, he has normal vital signs, normal cardiopulmonary exam, no spinal deformity, and a neurologic exam significant for decreased sensation and strength in the lower extremities. Two hours later, he lost complete sensation and motor function of his legs as well as rectal tone. Labs and CSF are unremarkable. Lumbar spine films are unremarkable. CT of the lumbar spine showed no evidence of spondylolysis, spondylisthesis, fracture, or soft tissue abnormalities. Brain MRI is normal. A non-contrast spine MRI (T1w sagittal image) four weeks later shows this:
Challenge: What happened?