Acute Febrile Multisystem Illness With Vasculitic Features in a Young Adult: A Diagnostic Challenge Between Infectious and Autoimmune Etiologies.

Acute febrile illness with rash and multisystem involvement in a young adult has a broad differential diagnosis spanning infectious and autoimmune etiologies. We report the case of a 21-year-old male who presented with fever, a mixed-morphology rash (petechiae and palpable purpura with centripetal spread), peripheral neuropathy, polyarthritis, and transaminitis following recent travel to Tennessee and Missouri. A prodromal cough two weeks prior to rash onset added diagnostic complexity. Initial serologic testing during hospitalization was negative for Rocky Mountain spotted fever (RMSF) and dengue, and confirmatory convalescent serologies remained unavailable at discharge. Despite early corticosteroid administration for suspected small-vessel vasculitis, no clinical improvement was observed. Empiric doxycycline was initiated, given the epidemiologic risk for tick-borne illness. This case highlights the critical importance of maintaining a broad differential, avoiding premature diagnostic closure, and prioritizing empiric antimicrobial therapy over early immunosuppression in diagnostically ambiguous, febrile multisystem illness.