Solitary Langerhans Cell Histiocytosis of the Femur in a Nine-Month-Old Girl.

Langerhans cell histiocytosis (LCH) is a rare clonal proliferative disorder of myeloid dendritic cells that can mimic infection or malignancy in paediatric bone lesions. A 9-month-old girl presented with a refusal to weight bear for two days. She was afebrile with mildly elevated inflammatory markers, including C-reactive protein (CRP) of 2 mg/L and white cell count (WCC) of 12×10⁹/L. Radiographs revealed an aggressive lytic lesion in the proximal femur with periosteal reaction. MRI showed a subperiosteal fluid collection suggestive of abscess, bone marrow edema, and adjacent muscle inflammation. The patient underwent curettage of the lesion. Histopathology confirmed LCH, with immunohistochemistry positive for cluster of differentiation (CD)1a, S100, CD68, and Langerin. Molecular analysis detected a V600D mutation. Whole-body MRI confirmed a solitary lesion, consistent with single-system unifocal bone LCH. No systemic therapy was initiated, and the patient was managed conservatively with observation. At six-week follow-up, she demonstrated significant clinical improvement and radiographic healing with no leg length discrepancy. This case highlights the importance of biopsy for diagnosing aggressive bone lesions in infants, as unifocal LCH can mimic osteomyelitis or malignancy. Early recognition and conservative treatment often result in an excellent outcome.