A 44-year-old woman suffers a fall while rock climbing, landing on her but tocks and falling forward. Despite prolonged airlift to the ED, she is hemodynamically stable. She complains of bilateral pain in her legs, distal to her knees. She has profound weakness in her bilaterally extensor hallucis longi and gastrocsoleus complexes and has marked saddle anesthesia. MRI shows a large, midline herniated disc, compressing each of the traversing nerve roots and entire cauda equina below its level, but sparing the exiting nerve roots. Which disc is most likely involved in this injuryinjury