A  

Plasmacytoma is essentially a plasma-cell neoplasm of adults; solitary bone plasmacytoma is exceedingly rare below 20 years of age, making it an unlikely cause of a lytic femoral lesion in a 10-year-old child [Tandon et al., 2022, PMID 34654755]. In contrast, brown tumours from hyperparathyroidism, metastatic deposits (e.g., neuroblastoma, leukemia), and Langerhans cell histiocytosis are all recognized pediatric causes of lytic bone lesions.