An 85-year-old woman was admitted to the hospital with a left femoral neck fracture and was confined to bed rest. On admission, muscle strength and sensation in the left lower limb were normal, but the next day, she was unable to move her left foot upwards. On examination, she was unable to dorsiflex her left ankle, with decreased sensation on the dorsal aspect of her left foot. The left

lower limb was externally rotated.

Select the most likely cause of her decreased ankle movement.