**Core Concept**
Hip fractures are a common injury in older adults, often resulting from low-energy trauma such as falls. The presentation can vary depending on the location and severity of the fracture.

**Why the Correct Answer is Right**
In this case, the patient's inability to walk or bear weight on her left hip, combined with external rotation of the left lower limb, and tenderness over the hip joint, are classic signs of a hip fracture. The inability to perform active straight leg raising (ASLR) further supports this diagnosis, as pain from a hip fracture can limit hip flexion. The mechanism of injury, a fall, also increases the likelihood of a hip fracture.

**Why Each Wrong Option is Incorrect**
* **Option A:** While osteoporosis is a risk factor for hip fractures, the presence of a hip fracture is not a direct consequence of osteoporosis itself.
* **Option B:** A hip dislocation would typically present with a more dramatic deformity and may not be as painful to move as a fracture.
* **Option C:** A hip strain or sprain may cause pain and stiffness, but would not typically result in an inability to bear weight or perform ASLR.

**Clinical Pearl / High-Yield Fact**
Hip fractures are a significant cause of morbidity and mortality in older adults, with a high risk of subsequent complications, including deep vein thrombosis, pressure sores, and pneumonia.

**Correct Answer: C. Hip fracture.**