## **Core Concept**
The patient is presenting with thigh pain after long-term treatment with Alendronate, a bisphosphonate used for osteoporosis management. Bisphosphonates, especially with prolonged use, are associated with an increased risk of atypical femoral fractures (AFFs). Early detection and management of such conditions are crucial.

## **Why the Correct Answer is Right**
The correct answer involves performing an investigation that can help identify the cause of thigh pain in a patient on long-term bisphosphonate therapy. Given the association between bisphosphonates and atypical femoral fractures, the most appropriate next step would be to perform imaging that can visualize the femur. An **X-ray of the thigh** would be the initial imaging modality of choice as it is readily available, cost-effective, and can provide immediate information about the bone structure, including signs of stress fractures or other abnormalities such as a femoral shaft fracture or cortical thickening, which are indicative of atypical femoral fractures.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While an MRI can provide detailed images of soft tissues and bone marrow, it is not typically the first-line imaging modality for evaluating bone pain or suspected fractures due to its higher cost and limited availability compared to X-ray.
- **Option B:** A bone scan can be useful for detecting stress fractures or metastatic disease but is not the first-line investigation for localized thigh pain in this context, as it does not provide as much detail about the bone structure as an X-ray and involves radiation.
- **Option C:** A CT scan provides more detailed images than an X-ray and can be useful for evaluating bone structure, but it is not the first-line investigation for this scenario due to higher radiation exposure and cost compared to an X-ray.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients on long-term bisphosphonate therapy are at risk of atypical femoral fractures. Clinicians should have a low threshold for investigating thigh pain in such patients. A characteristic "dull ache" in the thigh is a red flag symptom. Early recognition and X-ray evaluation can lead to timely intervention and prevention of complete fractures.

## **Correct Answer:** . X-ray of the thigh.