Psoriatic arthritis is a chronic inflammatory disease associated with psoriasis, and its diagnosis can be challenging owing to nonspecific symptoms, absence of reliable biomarkers, and occasional delay in skin manifestations. Herein, we report a case of psoriatic arthritis that initially presented as an acute finger inflammation mimicking infection. A 46-year-old woman developed sudden swelling and pain in the left ring finger during chemotherapy for cervical cancer. Based on the results of the physical examination, laboratory tests, and magnetic resonance imaging, pyogenic flexor tenosynovitis was suspected, and synovectomy was performed; however, bacterial and mycobacterial cultures yielded negative results. Despite the administration of antibiotics, the inflammation persisted, and she was referred to the Rheumatology Department, where she was diagnosed with reactive arthritis secondary to Chlamydia infection. Although the inflammation improved after antimicrobial therapy, the finger swelling persisted. Follow-up magnetic resonance imaging and serological testing were performed, and the patient was diagnosed of seronegative rheumatoid arthritis. Four years after onset, erythematous skin lesions appeared, and dermatological evaluation confirmed plaque psoriasis; thus, a definitive diagnosis of psoriatic arthritis was established. Disease-modifying antirheumatic drug adjustments improved symptoms, but residual 'pencil-in-cup' deformity and limited finger motion remained. This case highlights the difficulty in diagnosing psoriatic arthritis when arthritis precedes skin lesions. Clinicians should consider psoriatic arthritis in persistent or refractory arthritis and carefully monitor skin and nail changes to achieve an earlier diagnosis and prevent irreversible joint damage.
