MSFinals-2736

A 25-year-old patient visits the GP complaining of lower back pain and stiffness that extends to the buttocks for the past 3 months. The pain is most severe upon waking up, but cycling seems to alleviate it. The patient denies any injury but is an avid cyclist. Additionally, the patient experiences fatigue. The patient had Chlamydia and was treated with doxycycline 8 months ago. The patient has a history of anxiety and does not take any regular medication, but ibuprofen helps alleviate the pain. What is the most probable diagnosis?