The grаndpаrents оf а 22-mоnth-оld were called by the daycare for a report of the child refusing to bear weight after nap time. The child has had no known recent illnesses or injuries. He played well with his classmates and ate all of his lunch before napping in the crib. On exam, the child refuses to bear weight to the left leg and bats your hand away furiously. There is no obvious swelling, deformity, or redness, no concerning bruises or patterns of injury, and he is afebrile. A lower extremity radiograph is read, on the day of injury, as Normal by the Radiologist. Based on the child's age and history, which of the following conditions would you most likely suspect?