Musculoskeletal Injury (e.g., strain, sprain, or fracture): This is the most likely diagnosis due to the direct trauma to the arm during the motor vehicle accident (MVA). The force of the impact can cause injury to the muscles, ligaments, or bones in the arm.

Nerve Injury (e.g., brachial plexus injury or radial/ulnar nerve injury): The force of the MVA can cause stretching or compression of the nerves in the arm, leading to numbness, tingling, or pain.
Soft Tissue Injury (e.g., contusion or hematoma): The impact can cause bruising or bleeding into the soft tissues of the arm, leading to pain and swelling.
Vascular Injury (e.g., arterial or venous injury): Although less common, the MVA can cause injury to the blood vessels in the arm, leading to pain, swelling, or discoloration.

Compartment Syndrome: A potentially life-threatening condition that occurs when pressure within the muscle compartments increases, cutting off blood flow to the muscles and nerves. It requires prompt diagnosis and treatment to prevent permanent damage.
Vascular Injury with Active Bleeding: Uncontrolled bleeding from a vascular injury can lead to shock, organ failure, and death if not promptly addressed.
Spinal Cord Injury: Although the primary complaint is arm pain, it's essential to consider the possibility of a spinal cord injury, especially if there are symptoms such as numbness, tingling, or weakness in other parts of the body.

Complex Regional Pain Syndrome (CRPS): A rare condition characterized by chronic pain, inflammation, and sensitivity in the affected arm, often occurring after trauma.
Osteonecrosis: A rare condition where the blood supply to the bone is disrupted, leading to bone death and potentially causing arm pain.
Thoracic Outlet Syndrome: A rare condition where the nerves and blood vessels between the neck and shoulder are compressed, leading to pain, numbness, and tingling in the arm.