Gross Anatomy USMLE Step 1 Practice Question

A 42-year-old male sustains a cervical spine injury during a motor vehicle collision. He presents with acute respiratory distress, BP 89/54, HR 118, RR 28, and SpO2 78% on room air. CT cervical spine reveals a comminuted fracture with a bony fragment penetrating the spinal cord at C4. Neurological examination demonstrates complete loss of motor and sensory function below C3. He denies previous neck trauma. From which vertebral structure was this fragment most likely derived?