A 58-year-old man presents with a 3-month history of progressive left-sided leg pain radiating to the lateral foot and dorsum, with recent onset of foot drop and difficulty walking. On examination, ankle dorsiflexion is weak (grade 3/5). A lateral lumbar spine X-ray is obtained. The intervertebral disc space marked **B** (L4-L5) shows narrowing with a vacuum phenomenon visible. Based on the radiologic findings and clinical presentation, which of the following best explains the nerve root affected and the mechanism of compression?