Ans: A (Aneurysmal bone cyst) Ref: Apley's System of Orthopaedics and Fractures, 9th edition - TumoursExplanation:Aneurysmal Bone CystIt may be encountered at any age and in almost any bone, though more often in young adults and in the long-bone metaphyses.Usually it arises spontaneously but it may appear after degeneration or haemorrhage in some other lesion.With expanding lesions, patients may complain of pain.Occasionally, a large cyst may cause a visible or palpable swelling of the bone.X-rays show a well-defined radiolucent cyst, often trabeeulated and eccentrically placed.In a growing tubular bone it is always situated in the metaphysis and therefore may resemble a simple cyst or one of the other cyst-like lesions.Occasional sites include vertebrae and the flat bones.In an adult an aneurysmal bone cyst may be mistaken for a giant-cell tumour.Unicameral Bone CystAlso called as simple bone cyst, appears during childhood, typically in the metaphysis of one of the long bones and most commonly in the proximal humerus or femur.It is not a tumour, it tends to heal spontaneously and it is seldom seen in adults.The condition is usually discovered after a pathological fracture or as an incidental finding on X-ray.X-rays show a well-demarcated radiolucent area in the metaphysis, often extending up to the phvseal plate; the cortex may be thinned and the bone expanded.Hydatid CystIt is disease, is caused by the tapeworm Echinococcus.Parasitic infestation is common among sheep farmers, but bone lesions are rare.The bones most commonly affected are the vertebrae, pelvis, femur, scapula and ribs.The patient may complain of pain and swelling, or may present for the first time with a pathological fracture or compression of the spinal cord.Infestation sometimes starts in childhood but the cysts take so long to enlarge that clinical symptom and signs may not become apparent for many years.X-rays show solitary or nudtiloculated hone cysts, hut only moderate expansion of the cortices. However, cortical thinning may lead to a pathological fracture.In the spine, hydatid disease may involve adjacent vertebrae, with large cysts extending into the paravertebral soft tissues.These features are best seen on CT and MRI, which should always be performed if operative excision of the lesion is contemplated.Fibrous DysplasiaIt is developmental disorders in which areas of trabecular bone are replaced by cellular fibrous tissue containing Hecks of osteoid and woven bone.It may affect one bone (monostotic) one limb (monomelic) or many bones (polyostotic).If the lesions are large, the bone is considerably weakened and pathological fractures or progressive deformity may occur.The most common sites of occurrence are the proximal femur, tibia, humerus, ribs and craniofacial bones.Small, single lesions are asymptomatic.Large, monostotic lesions may cause pain or maybe discovered only when the patient develops a pathological fracture.Patients with polyostotic disease present in childhood or adolescence with pain, limp, bony enlargement, deformity or pathological fracture.Untreated, the characteristic deformities persist through adult life.Occasionally the bone disorder is associated with cafe-au-Iait patches on the skin and (in girls) precocious sexual development (Albright's syndrome).X-rays show radiolucent 'cystic' areas in the metaphysis or shaft; because they contain fibrous tissue with diffuse spots of immature bone, the lucent patches typically have a slightly hazy or 'ground-glass' appearance.The weight bearing bones may be bent, and one of the classic features is the 'shepherd's crook' deformity of the proximal femur.Radioscintigraphy shows marked activity in the lesion.