Swelling over distal end of femur with secondaries in lungs, diagnosis is:
**Core Concept**
Ewing's sarcoma is a malignant bone tumor that typically affects the long bones of children and young adults, often presenting with a painful swelling at the affected site. The tumor can also metastasize to distant organs, including the lungs.
**Why the Correct Answer is Right**
The correct diagnosis of Ewing's sarcoma is supported by the presence of a painful swelling at the distal end of the femur, which is a common location for this tumor. Additionally, the presence of secondaries in the lungs is consistent with the metastatic potential of Ewing's sarcoma. Ewing's sarcoma is characterized by the presence of small, round, blue cells that express CD99 and other specific markers, which can be confirmed through immunohistochemistry.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a malignant bone tumor, which is a key feature of Ewing's sarcoma.
* **Option B:** This option is incorrect because it mentions osteosarcoma, a different type of bone tumor that typically affects older children and young adults.
* **Option C:** This option is incorrect because it mentions a benign bone tumor, which is not consistent with the malignant nature of Ewing's sarcoma.
**Clinical Pearl / High-Yield Fact**
Ewing's sarcoma is a highly malignant tumor that requires prompt diagnosis and treatment to prevent metastasis and improve outcomes.
**Correct Answer: C. Osteosarcoma.**