Beyond Prosthetic Valve Infection: Anchoring Bias Delaying the Diagnosis of Synovial Sarcoma.

Synovial sarcoma is a rare soft tissue malignancy typically presenting as a deep-seated mass in younger individuals, with atypical presentations in older adults contributing to delayed diagnosis. We report a 68-year-old woman who developed persistent intermittent constitutional symptoms, prompting evaluation for fever of unknown origin (FUO) following surgical aortic valve replacement (SAVR), initially raising concern for prosthetic valve infection. Despite repeated negative blood cultures, an unremarkable echocardiogram, and a lack of objective evidence of infection, the diagnostic focus remained anchored in prosthesis-related etiologies, reinforced by coincidental clinical events and concern for hypersensitivity. Subsequent physical examination revealed a subtle deep-seated left thigh mass. Imaging demonstrated a heterogeneous lesion measuring approximately 7-8 cm with internal vascularity, and biopsy confirmed monophasic synovial sarcoma. This case highlights anchoring bias as a major contributor to diagnostic delay, with neoplastic fever, likely cytokine-mediated, mimicking chronic infection. It underscores the importance of maintaining a broad differential diagnosis in FUO and emphasizes the value of careful serial physical examination in identifying occult malignancy.