Delayed diagnosis of Ewing's sarcoma in a young patient presented with left knee monoarthritis.

A 19-year-old girl presented to the rheumatology clinic for left knee monoarthritis for the past 4 months. She also had constitutional symptoms with significant weight loss. On physical examination, she appeared cachexic, her left knee was swollen and tender. MRI of the left knee showed a soft tissue swelling extending into the knee joint. Left knee synovial fluid showed small round cells. Histopathology results were compatible with Ewing's sarcoma. Due to the delay in seeking medical advice, she succumbed to the disease 1 week after the diagnosis was made. Soft tissue/bone tumour causes monoarthritis is not common. A careful history taking, physical examination and investigations should be done in order to identify a sinister cause of monoarthritis such as Ewing's sarcoma. Early treatment should be initiated to ensure a better outcome.