A 20-year-old male complains of sudden onset shortness of breath. He is breathing at 24 times per minute with an oxygen saturation of 85%. What should you do?

Administering oxygen via a non-rebreather mask at 15 lpm is the most appropriate action in this scenario because the patient is exhibiting significant respiratory distress, as evidenced by his rapid breathing rate and low oxygen saturation. A non-rebreather mask is designed to deliver a high concentration of oxygen, which is crucial for patients with hypoxemia or severe respiratory distress.

In this case, the patient's oxygen saturation of 85% indicates that he is not receiving adequate oxygenation, and using a non-rebreather mask can help to quickly elevate his oxygen levels and improve his overall condition. This intervention provides immediate support for his breathing and can stabilize him while further assessments and treatments are initiated.

Other options, such as monitoring closely or using a nasal cannula, do not provide the same level of immediate oxygenation that is necessary for a patient with such low oxygen saturation. Encouraging deep breathing may not be effective given the patient's acute state and the risks associated with hypoxemia. Therefore, the most effective measure for improving this patient's oxygenation quickly is to administer oxygen via a non-rebreather mask.