Don't Miss Syphilis: A Mimicker of Lymphoma in an Era of Rising Incidence.

This report describes the case of a middle-aged man who presented with bilateral inguinal lymphadenopathy and an elevated serum level of soluble interleukin-2 receptor (sIL-2R), initially raising concern for malignant lymphoma. In Japan, sIL-2R is widely reimbursed as a diagnostic adjunct for lymphoma, which heightened the suspicion of malignancy in this case. However, serologic testing for syphilis returned positive, leading to a diagnosis of active syphilis. Lymph node biopsy was deferred, and the patient was treated with oral ampicillin, resulting in the resolution of lymphadenopathy and normalization of sIL-2R levels. This case underscores the importance of considering syphilis in the differential diagnosis of unexplained lymphadenopathy, particularly in regions where incidence is increasing.