Osteopetrosis.
Osteopetrosis is a rare hereditary bone disorder that presents in one of three forms: osteopetrosis tarda, osteopetrosis congenita and "marble bone" disease. Osteopetrosis tarda, the benign form, presents in adulthood, while the two more malignant variants, osteopetrosis congenita and marble bone disease, present in infancy and childhood, respectively. In all three forms, the main features are pathologic alteration of osteoclastic bone resorption and thickening of cortical and lamellar bones. Osteopetrosis tarda is usually discovered accidentally on routine radiographs and is often asymptomatic; however, patients may present because of related degenerative joint disease. Osteopetrosis congenita results in bone marrow failure and is almost always fatal. Marble bone disease causes short stature, cerebral calcification and mental retardation. Bone marrow transplant is the only chance for survival in patients with osteopetrosis congenita.