## **Core Concept**
The question tests knowledge of musculoskeletal tumors, specifically focusing on the diagnosis of a soft-tissue mass in the thigh with characteristic genetic findings. The clinical presentation and imaging findings suggest a soft-tissue tumor, and the genetic analysis points towards a specific diagnosis.

## **Why the Correct Answer is Right**
The correct answer, **D. Liposarcoma**, is supported by the clinical presentation of a large, deep soft-tissue mass in the thigh with an ill-defined border, which is consistent with a malignant tumor. The karyotypic analysis revealing t(12;16)(q13;p11) with amplification of the **MDM2 gene** is highly suggestive of a well-differentiated or dedifferentiated **liposarcoma**. This genetic abnormality is characteristic of liposarcomas, particularly the well-differentiated and dedifferentiated subtypes. Liposarcomas are malignant tumors of adipose tissue that can present as large, deep-seated masses in the extremities, such as the thigh.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but based on the context, we can infer that it would not match the genetic or clinical findings for liposarcoma.
- **Option B:** This option is not provided, but if it suggested a different type of sarcoma or tumor not associated with the MDM2 gene amplification or the specific translocation, it would be incorrect.
- **Option C:** This option is not provided, but similarly, if it did not align with the genetic findings or clinical presentation of a liposarcoma, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **MDM2 gene amplification** is a diagnostic hallmark for well-differentiated and dedifferentiated liposarcomas. This genetic feature can help differentiate liposarcomas from other types of sarcomas or benign lipomas. Clinicians and pathologists should consider this genetic testing in cases of suspected liposarcoma to guide diagnosis and treatment.

## **Correct Answer:** D. Liposarcoma