An 18-year-old college freshman presents with chronic fatigu…

An 18-yeаr-оld cоllege freshmаn presents with chrоnic fаtigue, as well as back pain and stiffness. He states the pain has been present for the past several months but appears to be worsening. His back symptoms are most severe at night and first thing in the morning and improve somewhat during the work day, when he is active. Based on this information, which of the following conditions is the most likely diagnosis for this patient?