Psoriatic arthritis.

Psoriatic arthritis (PsA) is an inflammatory arthritis associated with psoriasis. It is classified with the human leukocyte antigen B27 seronegative spondyloarthropathies because it shares certain features with other conditions included in that group. Specific features of PsA, however, distinguish it from both rheumatoid arthritis and other spondyloarthropathies. Although the cause and pathogenesis of PsA are not known, genetic, immunologic, and environmental factors are thought to play a role, and provide a rationale from therapeutic modalities.