Synovial sarcoma masquerading as juvenile idiopathic arthritis.

An early adolescent girl presented with chronic deforming monoarthritis. Initially misdiagnosed with skeletal tuberculosis, she received empirical treatment despite the absence of microbiological evidence. Her symptoms were managed intermittently with non-steroidal anti-inflammatory drugs, but the persistence and progression of her condition over the past 4 months raised suspicions of juvenile idiopathic arthritis. Her clinical presentation deteriorated, with severe knee arthritis and rapid calf swelling, prompting differential diagnosis including necrotising fasciitis or a ruptured Baker's cyst. Imaging confirmed synovitis with localised necrotising myositis, while persistent pain after fascial release and systemic inflammation without signs of infection indicated a more sinister underlying condition. Histopathological analysis of the biopsied tissue revealed synovial sarcoma, with subsequent PET-CT confirming skeletal and pulmonary metastasis. Despite timely diagnosis, the patient ultimately succumbed to her illness.