mrcem2-1045

A 45-year-old construction worker comes in with intense lower back pain. He has experienced back pain on and off for several years, but it has recently worsened significantly. He is walking with a noticeable limp, and during the examination, you observe that he has weakness in knee extension. You decide to conduct a digital rectal examination and discover that his anal sphincter tone is unexpectedly loose.

What is the SINGLE most probable diagnosis?