Polytrauma: This is the most likely diagnosis given the combination of a femur fracture and a chest injury, as it indicates multiple injuries that could have resulted from a high-energy trauma such as a car accident or a fall from a significant height.

Motor Vehicle Collision (MVC) Injuries: The pattern of injuries (femur fracture and chest injury) is consistent with those commonly seen in MVCs, where the force of the collision can cause fractures and internal injuries.
Fall from Height: A fall from a significant height could result in both a femur fracture due to the direct impact and a chest injury from either the initial impact or secondary injuries during the fall.

Fat Embolism Syndrome: Although not immediately apparent, a femur fracture increases the risk of fat embolism syndrome, which can be life-threatening. The presence of a chest injury could complicate or mask the symptoms of fat embolism.
Tension Pneumothorax: A serious and potentially life-threatening condition that can arise from chest injuries. It requires immediate recognition and treatment.
Hemorrhagic Shock: Significant blood loss from either the femur fracture or internal chest injuries could lead to hemorrhagic shock, a condition that is critical and requires immediate intervention.

Osteosarcoma with Pathologic Fracture and Metastasis: Although rare, a pathologic fracture of the femur could be the initial presentation of an underlying osteosarcoma, with the chest injury being a metastasis. This would be an unusual combination but should be considered in the differential diagnosis, especially if the patient's history and other findings suggest a possible malignancy.
Child Abuse: In pediatric patients, a combination of injuries such as a femur fracture and chest injury could be indicative of non-accidental trauma, which is a critical diagnosis not to miss due to its legal and protective implications.