You are seeing a 50-year-old male in your clinic who is complaining of lower back pain. He is complaining that this pain has been present for 6 months and that it is worse in the morning and improves with movement. He is also complaining that the symptoms are now waking him at night. You suspect a diagnosis of ankylosing spondylitis due to his past medical history of psoriasis and inflammatory bowel disease. What the is the most appropriate initial step to assist in the diagnosis of this condition?