A 32-year-old female equestrian had recently purchased new riding boots and a saddle for her horse. She had a long history of chronic low back pain and had been prescribed pain medication by her primary care physician. After a strenuous horseback riding session, during which her horse had performed well, she dismounted and experienced severe pain in her left leg, making it difficult to walk.
Upon examination, she exhibited a left foot drop, with the evertors and dorsiflexors of the left ankle measuring 2/5, while inversion of both ankles was 5/5. The straight leg-raising test was 80° on both sides, and tendon reflexes were normal. Pinprick sensation was reduced in the left foot.
What is the most likely diagnosis?