Ans- C Ref- Osteopetrosis, also known as Albers-Schonberg disease or marble bone disease, is an uncommon hereditary disorder that results from defective osteoclasts. Bones become sclerotic and thick, but their abnormal structure actually causes them to be weak and brittle. There are two separate subtypes of osteopetrosis: infantile autosomal recessive osteopetrosis benign adult autosomal dominant osteopetrosis Clinical presentation Presentation, in the majority of cases, is with a fracture due to the weakened bones. Fractures are often transverse with multiple areas of callus formation and normal healing. Additionally, there is crowding of the marrow, so bone marrow function is affected resulting in myelophthisic anaemia and extramedullary haematopoiesis with splenomegaly. This may terminate in acute leukaemia. Pathology Both forms are congenital abnormalities with localised chromosomal defects. These result in defective osteoclasts and overgrowth of bone. The bones become thick and sclerotic, but their increased thickness does not improve their strength. Instead, their disordered architecture results in weak and brittle bones. The osteoclasts lack carbonic anhydrase, which may play a role in the pathophysiology of the disease. Radiographic features The features are dependent on the subtype of osteopetrosis and are detailed in the individual articles: infantile autosomal recessive osteopetrosis benign adult autosomal dominant osteopetrosis Treatment and prognosis Treatment is with bone marrow transplant and resultant normalisation of bone production. Prognosis for the autosomal dominant adult subtype is good with a normal life expectancy. However, the autosomal recessive infantile subtype can result in stillbirth or death in infancy, with few patients living past middle age. History and etymology The term is derived from the Greek words 'osteo' meaning bone and 'petros' meaning stone. It was first described by German radiologist Heinrich Albers-Schonberg (1865-1921) in 1904 4. Differential diagnosis General imaging differential considerations include: heavy metal poisoning (e.g. lead) melorheostosis hypervitaminosis D pyknodysostosis fibrous dysplasia of skull or face