Q. A 69 year-old woman presents with low back pain and trouble with walking over the past few years. She has difficulty with prolonged standing but she finds that if she is in the grocery store, she can push the cart around for a long period of time and feels better. As well, she finds that it is easier to walk uphill than downhill. She has a known history of osteoarthritis, borderline diabetes mellitus, and coronary artery disease. She drinks alcohol socially, quit tobacco use in her twenties, and does not use illicit drugs. Temperature is 36.7 (98.7 F), Blood Pressure is 130/80, Heart Rate is 72 beats/minute. Neurological examination is normal except for mild reduced in sensation to cold temperature and pin-prick in the distal extremities and mild reduction in ankle reflexes, bilaterally, and symmetrically. Dorsalis pedis pulses and full and equal bilaterally in the feet. There is no tenderness to palpation across the spine and the straight leg raise test is negative. Cardiopulmonary auscultation is normal. Which of the following is the most likely cause her present complaint?