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This patient is experience reactive arthritis (also know as Reiter's Arthritis) which is a seronegative inflammatory arthritis that often follows a GU infection (like Chlamydia described in the history) or a bacterial GI infection. This group of seronegative inflammatory arthritides are associated with HLA B27. Also in this group are psoriasis, ankylosing spondylitis and IBD. HLA B8 is assoicated with Graves' disease and celiac sprue. HLA DR2 is associated with multiple sclerosis, SLE and Goodpasture's syndrome. HLA DR3 is associated with diabetes type I (especially in conjunction with HLA DR4). HLA DR5 is associated with pernicious anemia, and Hashimoto's thyroiditis.