A 35-year-old previously healthy male developed low-grade fever (37.5°C) and malaise for 1 day a week ago, followed by numbness and weakness in both feet that progressively worsened. Within 3 days, he developed complete paraplegia and fecal incontinence. Examination reveals complete paralysis of both lower limbs, hypotonia, bilateral negative Babinski sign, loss of pain/touch sensation and anhidrosis below the nipple level. Lumbar puncture shows normal cerebrospinal fluid (CSF), patent spinal canal, and unremarkable spinal X-rays. The correct diagnosis is: