This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: FMGE 2018, Subject name: Pediatrics
Marble bone disease is? 
Select and explain. 
A. Osteopetrosis, B. Osteomalacia, C. Osteosclerosis, D. Histiocytosis X
A: Osteopetrosis
Answer: A: Osteopetrosis 
Explanation: Osteopetrosis/ marble bone disease: * Bone within bone appearance on xray (increased bone density) * Due to mutation in CLCN7 gene. * Leads to defective bone resorption * C/F: Neurological problems present, Deafness, Macrocephaly , hepatosplenomegaly, Pancytopenia * Investigation: Low phosphorus and calcium levels with normal Vit D levels and elevated PTH.