**Core Concept**
Ewing's sarcoma is a malignant bone tumor that primarily affects children and young adults, characterized by a specific chromosomal translocation involving the EWSR1 gene. The prognosis of Ewing's sarcoma is influenced by various factors, including the tumor's location, size, and the presence of metastasis.

**Why the Correct Answer is Right**
The correct answer is associated with poor prognosis in Ewing's sarcoma because it indicates a high risk of metastasis and recurrence. The presence of metastasis at diagnosis is a significant prognostic factor, as it suggests that the tumor has already spread to distant sites, making it more challenging to treat. The metastatic spread of Ewing's sarcoma can occur through the bloodstream or lymphatic system, and the most common sites of metastasis include the lungs, bone marrow, and other bones.

**Why Each Wrong Option is Incorrect**
* **Option A:** While age is an important prognostic factor in many cancers, it is not directly associated with poor prognosis in Ewing's sarcoma. Younger patients are more likely to be diagnosed with Ewing's sarcoma, but age is not a primary determinant of prognosis.
* **Option B:** Tumor location is an important factor in Ewing's sarcoma, but it is not the primary determinant of prognosis. Tumors located in the pelvis or spine may be more challenging to treat, but they do not necessarily indicate poor prognosis.
* **Option C:** While tumor size can be an important prognostic factor in some cancers, it is not directly associated with poor prognosis in Ewing's sarcoma. The size of the tumor can affect treatment options and outcomes, but it is not the primary determinant of prognosis.

**Clinical Pearl / High-Yield Fact**
Patients with Ewing's sarcoma who present with symptoms such as bone pain, swelling, or weakness should be evaluated promptly, as delayed diagnosis can lead to increased risk of metastasis and poor prognosis. A high index of suspicion and prompt imaging studies, such as MRI or CT scans, are essential for early detection and treatment of Ewing's sarcoma.

**Correct Answer: C.**