Final answer:
The step not part of the scene size-up is 'Opening the airway of a patient who is in cardiac arrest', as scene size-up involves assessing the scene, not providing direct patient care.
Step-by-step explanation:
The step that is not part of the scene size-up is 'Opening the airway of a patient who is in cardiac arrest'. Scene size-up is a process primarily used by emergency responders, including those in pre-hospital settings like paramedics. Scene size-up includes establishing the number of patients, taking Body Substance Isolation (BSI) precautions, and evaluating the scene for safety hazards. Opening an airway is a direct patient care step that occurs after the scene has been deemed safe and comes under primary patient assessment.