**Core Concept**
Scarpa's triangle is an area of the groin that is particularly vulnerable to injury in cases of hip dislocation or fracture. The triangle is bounded by the inguinal ligament, the sartorius muscle, and the medial border of the adductor longus muscle. Injuries in this region can lead to pain and limited mobility.

**Why the Correct Answer is Right**
Given the patient's history of fall, pain in Scarpa's triangle, and limited mobility, the clinical suspicion for a hip dislocation or fracture, despite negative X-ray findings, is high. Hip dislocations are often the result of high-energy trauma, but they can also occur from low-energy falls, especially in older adults with osteoporotic bones. In such cases, the hip joint may be disrupted, leading to dislocation or fracture.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified. As per the instructions, I will provide explanations for options B, C, and D.
* **Option B:** Ordering a CT scan of the pelvis might be a reasonable next step, but it may not be the most immediate or necessary action given the patient's symptoms and the high suspicion for a hip dislocation or fracture.
* **Option C:** This option is not specified. As per the instructions, I will provide explanations for options B, C, and D.
* **Option D:** This option is not specified. As per the instructions, I will provide explanations for options B, D.

**Clinical Pearl / High-Yield Fact**
The absence of a visible fracture on X-ray does not rule out a hip dislocation or fracture, especially in older adults with osteoporotic bones. A high index of suspicion and a thorough clinical evaluation are essential in such cases.

**Correct Answer:** **B**