**Core Concept**
Retroperitoneal soft tissue sarcomas (RSTS) are rare, aggressive tumors that arise from the soft tissues in the retroperitoneal space. Accurate diagnosis and staging of RSTS are crucial for determining the optimal treatment plan and predicting patient outcomes. Imaging modalities play a vital role in the evaluation of RSTS, and each modality has its strengths and limitations.

**Why the Correct Answer is Right**
The investigation of choice for RSTS is **Contrast-Enhanced Computed Tomography (CECT)**. CECT provides excellent spatial resolution, allowing for detailed assessment of the tumor's size, location, and relationship with surrounding structures. Additionally, CECT can help identify the presence of lymphadenopathy and distant metastases. CECT is also useful for guiding biopsies and evaluating the response to treatment.

* CECT can help differentiate RSTS from other retroperitoneal pathologies, such as lymphoma, metastatic disease, and benign tumors.

**Why Each Wrong Option is Incorrect**
* **Option A:** Magnetic Resonance Imaging (MRI) is not the investigation of choice for RSTS. While MRI provides excellent soft tissue contrast, it is often limited by artifacts and is not as widely available as CECT.
* **Option B:** Positron Emission Tomography (PET) is useful for detecting distant metastases and assessing the response to treatment, but it is not the primary investigation for RSTS.
* **Option C:** Ultrasound is not suitable for evaluating RSTS due to the depth and complexity of the retroperitoneal space.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with suspected RSTS, it is essential to obtain a high-quality CECT scan to assess the tumor's extent and relationship with surrounding structures. This information will guide further management, including biopsy, surgical resection, and adjuvant therapy.

**Correct Answer: C. Contrast-Enhanced Computed Tomography (CECT)**