The Great Imitator Behind a Rare Mask: Secondary Syphilis Diagnosed During an Atypical Flare of Tumoral Calcinosis.

Tumoral calcinosis is a rare disorder of ectopic calcium phosphate deposition, usually involving periarticular soft tissues around large joints. Depending on etiology, it may be associated with hyperphosphatemia, fibroblast growth factor 23 pathway abnormalities, recurrent inflammation, ulceration, secondary infection, and progressive functional limitation. Syphilis, caused by Treponema pallidum, remains one of the classic infectious mimics in clinical medicine. Secondary syphilis may present with constitutional symptoms, rash involving the palms and soles, arthralgia, periostitis, tenosynovitis, and inflammatory arthritis, thereby overlapping with the presentation of rheumatic and metabolic bone diseases. We present a case of a 37-year-old man with longstanding nodular tumoral calcinosis who presented with fatigue, generalized arthralgia, worsening discomfort at established calcification sites, and painless palmar lesions. Because of his prior history of tumoral calcinosis-associated ulceration and superficial infection, the initial clinical frame favored a typical tumoral calcinosis flare. Serologic testing, performed because of systemic symptoms and unusual palm lesions, revealed a positive Rapid Plasma Reagin (RPR) and titer of 1:32 with positive Treponema pallidum antibody. Directed sexual history elicited recent unprotected exposures. The patient was diagnosed with secondary syphilis transitioning into early latent syphilis and was treated with doxycycline 100 mg twice daily for 14 days as Penicillin G Benzathine was not available because of supply shortage. This case report highlights diagnostic overshadowing, anchoring bias, and the need to preserve a broad infectious differential when patients with established rheumatologic or metabolic disease develop atypical systemic features.