Final answer:
The most appropriate next step is to secure the patient's airway.
Step-by-step explanation:
The most appropriate next step for a trauma patient presenting with inspiratory stridor and a suspected c-spine injury, along with an oxygen saturation of 88% on high-flow oxygen via a nonrebreathing mask, would be to secure the patient's airway.
One possible cause for the inspiratory stridor in this scenario could be a compromised airway due to the suspected c-spine injury. To ensure the patient's airway is maintained and optimally supported, an emergency intubation or the insertion of an advanced airway device may be necessary.
Once the airway is secured, further assessment and management can be done to address the underlying cause of the stridor and hypoxemia.