**Core Concept:** Alendronate is a bisphosphonate used for the treatment of osteoporosis by inhibiting osteoclast activity and reducing bone resorption. Common side effects include gastrointestinal complaints and musculoskeletal pain.

**Why the Correct Answer is Right:** In this case, the patient is on long-term Alendronate treatment for osteoporosis, which increases the likelihood of musculoskeletal pain as a side effect. The pain is localized to the right thigh, suggesting a possible stress fracture due to the weakening of bone tissue caused by Alendronate.

**Why Each Wrong Option is Incorrect:**

A. **X-ray:** While X-rays can detect fractures, they are not sensitive enough to detect early-stage stress fractures. The pain may be present before the fracture is visible on X-ray.

B. **Laboratory Tests:** Laboratory tests are not relevant for diagnosing musculoskeletal pain due to Alendronate treatment. They are used to assess bone mineral density, renal function, and other general health markers.

C. **Ultrasound:** Ultrasound is not commonly used for diagnosing musculoskeletal pain, as it is less sensitive than X-rays for detecting early-stage fractures.

D. **Investigation:** Investigation is the correct term for the procedure we are discussing. Other investigations are not relevant to this question.

**Clinical Pearl:** In patients taking Alendronate for osteoporosis, a history of prolonged use, and the presence of musculoskeletal pain should raise the suspicion of a stress fracture. Early detection and management of such fractures are crucial to prevent complications and further bone damage.

**Correct Answer:** C. **Ultrasound:** Although less sensitive than X-rays, ultrasound can be useful in detecting early-stage fractures where X-rays are negative. It is commonly used in pregnant women or patients with renal insufficiency, where X-rays are contraindicated. However, the focus of this question is to identify a suitable imaging modality for this patient. Therefore, the correct answer is ultrasound (option C) as it is a suitable alternative to X-rays for imaging musculoskeletal pain in this context.