This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
[{'content': 'Histiocytosis X is seen in except : \nSelect and explain. \nA. Hand schuller Christian disease, B. Eosinophilic granuloma, C. Letter-siwe syndrome, D. Torres syndrome', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Langerhans cell histiocytosis (Histiocytosis X) :\n\nHistocytosis X is characterized by proliferation of Langerhans histiocytes (tissue macrophages).\nIt is subdivided into three categories : -\n\n\nLetterer - Siwe syndrome.\nHand -Schuller-Christian disease.\nEosinophilic granuloma..\n\n\nTumor cells in each are derived from dendritic cells and express S - 100, CD1a and HLA - DR.\nThe presence of Birbeck granules in the cytoplasm is characteristic.\nUnder the electron microscope, Birbeck granules have a pentalaminar, rodlike, tubular appearance and sometimes a dilated terminal end (tennis - racket appearance).', 'role': 'assistant'}]