MUC5AC and Arthritis: The most frequently involved joints are knees, followed by ankles, wrist, and elbows.17 Back pain is rare and sacroiliac joint involvement is not part of the disease.18 Patients with BS and arthritis also have more acne lesions.19 Furthermore, patients with arthritis and acnea lesions have a significantly more entesopathy scores.20 Synovial fluid is commonly inflammatory but has a good mucin clot.