<prompt> Marble bone disease is?OsteopetrosisOsteomalaciaOsteosclerosisHistiocytosis X  <response>: Osteopetrosis marble bone disease: Bone within bone appearance on xray increased bone density Due to mutation in CLCN7 gene. Leads to defective bone resorption CF: Neurological problems present, Deafness, Macrocephaly , hepatosplenomegaly, Pancytopenia Investigation: Low phosphorus and calcium levels with normal Vit D levels and elevated PTH. 