Which of the following actions should be taken first when treating a woman with vaginal bleeding?

Prioritizing the patient's airway is essential in any medical emergency. If a patient is experiencing significant vaginal bleeding, there is a potential for shock or a decrease in consciousness, which could compromise airway integrity. Ensuring the airway is clear allows for effective respiratory function, which is crucial, especially if the patient shows any signs of deterioration or altered mental status.

Taking this action first ensures that if the patient has low blood volume due to bleeding and begins to show symptoms like fainting or weakness, their ability to breathe and maintain adequate oxygenation is protected. Only after confirming that the airway is clear can other crucial interventions like controlling bleeding, assessing vital signs, and establishing intravenous access be performed safely and effectively.