The most likely diagnosis is psoriatic arthritis. Although estimates of the prevalence of psoriatic arthritis in patients with psoriasis vary, more recent studies using standardized diagnostic criteria indicate that psoriatic arthritis is present in approximately 15% to 20% of those with psoriasis. Patients who have features consistent with psoriatic arthritis should be examined closely for psoriasiform skin lesions on the umbilicus, gluteal cleft, extensor surfaces, posterior auricular region, and scalp. Nails should be examined for pitting or onycholysis. Characteristic features of psoriatic arthritis include enthesitis, dactylitis, tenosynovitis, arthritis of the distal interphalangeal joints, asymmetric oligoarthritis, and spondylitis. The recently developed Classification Criteria for Psoriatic Arthritis (CASPAR) have a sensitivity and specificity of more than 90%, especially for the diagnosis of early psoriatic arthritis. This patient fulfills the CASPAR criteria because she has inflammatory articular disease with psoriasis, psoriatic nail dystrophy, dactylitis, and a negative rheumatoid factor.

This patient does not have symptoms or findings of inflammatory back pain associated with ankylosing spondylitis; her back pain is related to use and improves with rest, which is noninflammatory. HLA-B27 positivity alone is insufficient to diagnose this disease, and peripheral articular disease is not typical for ankylosing spondylitis.

Nearly 50% of patients with inflammatory bowel disease (IBD) develop musculoskeletal symptoms. Peripheral arthritis may be acute and remitting with a pauciarticular distribution commonly involving the knee. Peripheral arthritis can also be chronic or relapsing, with prominent involvement of the metacarpophalangeal joints and less correlation with intestinal inflammation. IBD-associated arthritis is also unlikely because this patient has no symptoms of bowel disease.

Reactive arthritis (formerly known as Reiter syndrome) is a postinfectious arthritis triggered by infections causing urethritis or diarrhea, although patients may be asymptomatic. Arthritis, usually oligoarticular, develops several days to weeks after the infection. Reactive arthritis can cause dactylitis; however, this patient has no history of a preceding infection, making this an unlikely diagnosis.