MSFinals-5941
A 25-year-old man with inflammatory bowel disease has been experiencing lower back pain, stiffness, and buttock pain for the past six months. He notices that his pain improves after playing squash on Saturdays. He has tested positive for HLA-B27 and his blood tests, including C-reactive protein and erythrocyte sedimentation rate, are normal except for a mild hypochromic microcytic anemia. What is the most likely diagnosis?