Final answer:
The correct action to take for an 18-year-old female experiencing respiratory distress, shallow breathing, and cyanosis after blunt chest trauma, with a low blood pressure and rapid thready pulse, is to provide ventilation assistance with a bag valve mask.
Step-by-step explanation:
Following blunt trauma to the chest, an 18-year-old female presents with respiratory distress, shallow breathing, and cyanosis. Additionally, she has a blood pressure of 80/50 mm Hg and a pulse of 130 beats/min which is thready. Given the emergency nature of her symptoms, the correct initial action would be to B. provide ventilation assistance with a bag valve mask. This choice is based on the urgent need to address her inadequate ventilation and poor oxygenation, which are indicated by her current state including respiratory distress and cyanosis.
Performing a rapid head-to-toe physical assessment (Option A) can be considered after ensuring that the patient's breathing is supported. Applying 100% oxygen with a nonrebreathing mask (Option C) is appropriate if the patient can breathe adequately on her own, which is not the case here. Placing her supine and elevating her lower extremities (Option D) might be considered if the patient is experiencing shock, but it does not directly address her current distressing breath-related symptoms and the attendant risk of hypoxia.