Ans. is 'c' i.e. Ewing's Sarcoma Small round cells rich in PAS positive diastase sensitive granules confirms the diagnosis of Ewing's sarcoma.The histopathological features of Ewing's sarcoma are:Small round cells having scant cytoplasmThese cells are rich in glycogen* this glycogen filled cytoplasm is detected by staining with periodic acid - shiff (PAS +ve)These cells are diastase sensitive. Diastase is an enzyme which splits glycogen.Homer wright rosettes* (the tumor cells are arranged in circles around a central fibrillary space)Remember: The other round cell tumours of boneOsteosarcomaReticulum cell sarcomaMetastatic neuroblastomaAlso RememberGross pathological features of Ewing's sarcoma:The tumour arises in the medullary cavity from the endothelial cells.New bone formation may extend along the shaft and sometimes it appears as fusiform layers of bone around the lesion, (onion peel effect)*The tumour erodes the cortex early often extending into the soft tissues with radiating streaks of ossification and reactive periosteal bone at the proximal and distal margins, (these features the 'sunray appearance' and codman's triangle are usually associated with osteosarcoma but they are just as common in Ewing's sarcoma)Clinical features of Ewing's sarcomaIt occurs most commonly between the age of 10-20 years especially in the tibia, fibula or clavicle.The patient presents with pain and swelling often arising in the diaphysis Sometimes fever is also associated with it confusing it with osteomyelitis