A 7-month-old previously healthy girl was brought to the emergency department with high-grade fever and refusal to move her left lower limb. She had been recently treated for a febrile urinary tract infection caused by E. coli (sensitive to cefuroxime) at another hospital. She was referred for further evaluation to rule out malignancy or leukemia. On arrival, she was alert and hemodynamically stable, with no visible rashes, ulcers, or wounds. Her left knee was visibly swollen, warm, and tender on palpation. Initial investigations revealed leukocytosis (WBC: 26,000/μL), thrombocytopenia (platelet count: 966,000/μL), a markedly elevated CRP (2,552 mg/L), an ESR of 115 mm/hr, and an LDH of 230 IU/L. Blood cultures were negative. Coombs test and bone marrow biopsy ruled out hematological malignancies. Left knee X-ray showed joint capsule expansion. An arthrotomy was performed, revealing turbid synovial fluid with numerous white blood cells (WBCs); however, cultures remained negative. She was treated with IV antibiotics (initially linezolid and meropenem, later changed to teicoplanin and cefepime), along with IVIG and methylprednisolone, leading to marked clinical improvement. She was discharged after completing a 3-week course of IV antibiotics. One day after stopping steroids, she returned with fever, tenderness, and swelling in the right knee. Laboratory evaluation revealed hemoglobin of 8.6 g/dL, WBC count of 14,300/μL, platelet count of 430,000/μL, CRP of 244.7 mg/L, ESR of 87 mm/hr, LDH of 246 IU/L, and ferritin of 401 µg/L. MRI of the right knee revealed synovial thickening and inflammation. Arthrotomy revealed thick, turbid synovial fluid. Synovial fluid analysis showed a WBC count of 79,950/μL (predominantly neutrophils), and culture grew Bacillus subtilis, confirming the diagnosis of septic arthritis of the right knee.