<prompt> Soap bubble appearance is seen in:Simple unicameral cystAneurismal cystGiant cell tumourOsteogenic sarcoma  <response>: Cl Giant cell tumour: Ref: 181-AGiant c ell tumourSoap bubble appearance due to ridging of the surrounding bonesThe appearance of a cystic lesion in mature bone, extending right to the subchondral plate is so characteristic that the diagnosis is seldom in doubt.Aneurysmal bone evst: - well defined radio lucent cyst, often trabeculated and eccentrically placed. It usually docs not extend right upto the articular margin.Osteochondroma: - Large lesions show cauliflower appearance with degeneration and calcification in the centre of the cartilage cap.Chondroblastoma: - well demarcated radiolucent area in epiphysisChondromvxoid fibroma: - Rounded or ovoid radio lucent area placed eccentrically in the metaphysis. The endosteal margin may be scalloped, but is almost always bounded by a dense zone of reactive bone extending tongue - like towards the diaphysis.Chondroma Enchondroma: - Centrally placed radiolucent area at the junction of metaphysis and diaphysis. In mature lesions there are flecks or wisps of calcification within the lucent area Pathognomic featuresOsteoblastoma Giant osteoid osteoma: well demarcated osteolytic lesion which may contian small flecks of ossification radio isotpe scan will reveal the hot area.Osteoidosteoma: - A small radiolucent area, the so called Nidus lesion in the diaphysis are surrounded by dense sclerosis and cortical thickening. 