What is the most likely cause of leg pain, swelling, and warmth in a 7-year-old boy with an "onion skin" appearance of the periosteum?

• A. Multiple myeloma
• B. Septic arthritis
• C. Ewing's sarcoma
• D. Enchondroma

Answer: (C)

The most likely cause of leg pain, swelling, and warmth in a 7-year-old boy with an "onion skin" appearance of the periosteum is Ewing's sarcoma. This condition is a type of aggressive bone tumor that primarily affects children and adolescents. The "onion skin" appearance refers to the characteristic radiographic finding of layers of periosteal reaction that resemble onion layers, which is commonly associated with Ewing's sarcoma.

Ewing's sarcoma typically presents with localized pain, swelling, and warmth in the affected area due to the tumor's aggressive nature, leading to inflammation and periosteal reaction. The age of the child and the specific radiographic finding are significant indicators that support the diagnosis of Ewing's sarcoma.

While other conditions listed could cause leg pain and swelling, they do not present with the characteristic periosteal changes associated with Ewing's sarcoma. For example, multiple myeloma typically occurs in older adults and is characterized by different symptoms and radiographic findings. Septic arthritis usually presents with acute symptoms and joint involvement but does not specifically exhibit an "onion skin" appearance. An enchondroma is a benign cartilage tumor and would not typically cause the same aggressive symptoms or peri