A 52-year-old man presents with acute onset bilateral leg weakness, saddle anesthesia, and urinary retention. MRI of the lumbosacral spine shows a large central disc herniation compressing the structure marked **A** (conus medullaris) at the L1-L2 level. On examination, he has symmetric lower limb weakness with hyperreflexia in the knees and ankles, brisk plantar responses, and severe early-onset bowel/bladder dysfunction. Which clinical syndrome best explains this constellation of findings?