What condition is associated with an "onion-skin" periosteal reaction?

The condition associated with an "onion-skin" periosteal reaction is indeed Ewing's sarcoma. This specific imaging finding often observed in radiographs refers to the appearance of the periosteum (the membrane enveloping the bones) that displays layers resembling an onion, indicative of a specific type of bone growth pattern.

Ewing's sarcoma is a type of aggressive bone tumor that predominantly affects children and adolescents. The characteristic onion-skin appearance results from reactive changes in the periosteum due to the fast-growing tumor, which elevates the periosteum and leads to the deposition of new layers of bone. As the tumor evolves, these layers create the distinctive "onion-skin" appearance when viewed on imaging studies.

In contrast, other conditions listed do not typically produce this radiological sign. Osteosarcoma can show different periosteal reactions, often with more aggressive bone destruction patterns. Osteomyelitis may lead to a more diffuse periosteal elevation but is not typically described as onion-skin. A pilonidal cyst is soft tissue-related and does not involve periosteal reactions as seen in bone tumors.