Ankle Instability and Sprains

A 30-year-old woman is on her first ski trip. While skiing, she fell and twisted her left leg. She cannot stand on her left leg because of pain. On examination, the left knee is swollen and tender. With the patient sitting on the stretcher with her knee flexed, the doctor on duty performed a test which came out to be positive. What is the most likely injured structure?

Which imaging modality is LEAST useful in the initial diagnosis of stress fractures?