## **Core Concept**
The question presents a clinical scenario suggestive of a bone tumor or infection, specifically Ewing's sarcoma, given the patient's age, symptoms, and radiographic findings. The laminated periosteal reaction on X-ray is a characteristic feature of this condition.

## **Why the Correct Answer is Right**
The correct answer, **MRI of the thigh**, is the next best line of investigation because MRI provides detailed images of soft tissues and bone marrow, which are essential for evaluating the extent of the lesion, its relationship with surrounding structures, and for guiding biopsy if needed. Ewing's sarcoma often presents with a permeative bone destruction and a soft tissue mass, which can be better visualized on MRI compared to CT.

## **Why Each Wrong Option is Incorrect**
- **Option A (CT of the thigh):** While CT can provide information on bone destruction and periosteal reaction, it is less sensitive than MRI in evaluating the soft tissue component of the tumor and bone marrow involvement.
- **Option B (Bone scan):** A bone scan can show areas of increased bone turnover but lacks the specificity and detail needed for surgical planning or diagnosis.
- **Option D (Biopsy of the lesion):** Although biopsy is crucial for definitive diagnosis, it is an invasive procedure and typically follows imaging studies that help in planning the biopsy approach.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl in this scenario is to consider Ewing's sarcoma in the differential diagnosis of a child or young adult presenting with a painful swelling and systemic symptoms like low-grade fever. The presence of a laminated or "onion-skin" periosteal reaction on X-ray is highly suggestive of Ewing's sarcoma.

## **Correct Answer:** .