Ans: D (Osteopetrosis) Ref : Apley's System of Orthopaedics ami Fractures, 9th edition, pg. 131-136, 167Explanation:Almost half the bone volume is mineral matter - mainly calcium and phosphate in the form of crystalline hydroxyapatite which is laid down in osteoid at the calcification front.In mature bone the proportions of calcium and phosphate are constant and the molecule is firmly bound to collagen.It is important to appreciate that in life 'demineralization' of bone occurs only by- resorption of the entire matrix.OsteoblastsThese cells are concerned w ith bone formation and osteoclast activation.They are derived from mesenchymal precursors in the bone marrow and the deep layer of the periosteum.Differentiation is controlled by a number of interacting growth factors, including bone morphogenetic proteins.They are responsible for the production of type I collagen as well as the non-collagenous bone proteins and for the mineralization of bone matrix.OsteoclastsThese are large mu hi nucleated cells and are the principal mediators of bone resorption.They develop from mononuclear precursors in the haemopoietic marrow' (the same lineage as macrophages) under the influence of local osteoblastic stromal cells that generate an essential osteoclast differentiating factor - receptor activator of nuclear faetor-KfS ligand (RANKL ) - w'hich binds with a specific receptor site (RANK) on the osteoclast precursors.OsteoporosisIt is characterized by an abnormally low bone mass, normal mineral to matrix ratio and defects in bone structure, a combination which renders the bone unusually fragile and at greater than normal risk of fracture in a person of that age, sex and race.In X-rays characteristic signs of osteoporosis are loss of trabecular definition, thinning of the cortices and insufficiency fractures.Compression fractures of the vertebral bodies, wedging at multiple levels or biconcave distortion of the vertebral end-plates due to bulging of intact intervertebral discs are typical of severe pos tmenopau sal osteoporos i s.OsteomalaciaIt is a manifestation of Vitamin D deficiency in adults.There is inadequate mineralisation of hone.It is characterized by a decreased ratio of hone mineral to matrix. Thus there is a decrease in bone mineral mass.HyperparathyroidismIt is characterized by a general loss of bone substance.In severe cases, osteoclastic hyperactivity produces subperiosteal erosions, endosteal cavitation and replacement of the marrow spaces by vascular granulations and fibrous tissue (osteitis fibrosa cystica).Haemorrhage and giant-cell reaction within the fibrous stroma may give rise to brownish, tumour-like masses, whose liquefaction leads to fluid-filled cysts.OsteopetrosisIt is characterized by sclerosis and thickening of the bones which appear with increased radiographic density.This is the result of an imbalance between bone formation and bone resorption: in the most common form, osteopetrosis, there is failed bone resorption due to a defect in osteoclast production and/or function.As a result of this bone mass is increased in this condition.This does not translate into increased strength but causes increased brittleness and susceptibility to pathological fractures.This condition is also called marble bones disease or Albers-Schonberg Disease.