**Core Concept**
The patient's presentation of hip pain and tenderness in Scarpa's triangle, despite a normal X-ray, suggests a possible hip labral tear or osteonecrosis, which may not be visible on plain radiographs. The underlying principle here is the need for further imaging to diagnose these conditions, which require detailed visualization of soft tissue structures.

**Why the Correct Answer is Right**
An MRI is the most appropriate next investigation for several reasons. Firstly, it provides excellent visualization of soft tissues, including the labrum, cartilage, and surrounding muscles and tendons. This is crucial for diagnosing conditions like labral tears, which are often difficult to detect on other imaging modalities. Secondly, an MRI can also detect early signs of osteonecrosis, such as marrow edema, which may not be apparent on X-ray. The high-resolution images obtained with MRI enable accurate diagnosis and planning of treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** USG guided aspiration is not the next investigation, as it is typically used for diagnosing or treating conditions like joint effusions or septic arthritis, which are not suspected in this case.

**Option B:** CT scans are not the best choice for evaluating soft tissue structures like the labrum or cartilage, although they can provide useful information on bone architecture and joint alignment.

**Option D:** Bone scans are not typically used for diagnosing conditions like labral tears or osteonecrosis, as they are more sensitive to changes in bone metabolism, which may not be present in these conditions.

**Clinical Pearl / High-Yield Fact**
When evaluating hip pain with a normal X-ray, always consider the possibility of a labral tear, osteonecrosis, or other soft tissue injuries, and request an MRI to visualize these structures accurately.

**✓ Correct Answer: C. MRI. MRI is the most appropriate next investigation for this patient.**