Mr. F is a 65-year-old man with type 2 diabetes, hypertension, and osteoarthritis who comes into your office complaining of several months of low back pain. Sometimes the pain is limited to his back, but it sometimes radiates to his buttocks, hips, thighs and calves when he walks. Although generally achy in character, he sometimes feels numbness in both thighs. The pain gets better when he sits down, although he finds it also goes away while he is grocery shopping if he bends a bit to push the cart. He does not have pain while in bed, and he has more pain standing than sitting. Over-the-counter ibuprofen helps somewhat, but he feels quite limited in his activity. He has no fever, history of instrumentation, or injection drug use.
At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis?