**Core Concept**
The question is testing the molecular characteristics of small round cell tumors, specifically those presenting as a tibial mass in a pediatric patient. The correct answer will be related to the genetic or molecular alterations associated with these tumors.

**Why the Correct Answer is Right**
Small round cell tumors in children, such as Ewing's sarcoma, are characterized by the presence of a specific chromosomal translocation. The most common translocation in Ewing's sarcoma is t(11;22)(q24;q12), which results in the fusion of the EWSR1 gene with the FLI1 gene. This fusion leads to the formation of an aberrant transcription factor that contributes to the development and progression of the tumor. The presence of this translocation is a hallmark of Ewing's sarcoma and is often used as a diagnostic marker.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a known molecular alteration associated with small round cell tumors in children.
* **Option B:** This option is incorrect because it is a common translocation in osteosarcoma, not small round cell tumors.
* **Option C:** This option is incorrect because it is a common mutation in neuroblastoma, not small round cell tumors of the tibia.
* **Option D:** This option is incorrect because it is a common mutation in rhabdomyosarcoma, not small round cell tumors of the tibia.

**Clinical Pearl / High-Yield Fact**
The presence of a small round cell tumor in a pediatric patient should raise suspicion for Ewing's sarcoma, particularly if there is a tibial mass. The diagnosis can be confirmed by the presence of the EWSR1-FLI1 fusion gene and specific histopathological features.

**Correct Answer: A. t(11;22)(q24;q12)**