a 64 year old man comes to the office complaining of back in leg pain that is aggravated by walking. he tells you that the pain starts after he walks two blocks. he gets relief when he is sitting or when he is walking and leaning forward. physical exam reveals pain and decreased motion with backward bending of the lumbosacral spine. there is a negative straight leg raise test result and no neurological deficits. what is the most likely diagnosis?