FCEM2-1805

A 32-year-old woman presents with a history of increased wheezing over the past two days. She suffers from seasonal allergies in the spring months, which has been worse than usual over recent weeks. When auscultating her chest, you can hear scattered polyphonic wheezes. Her peak flow at presentation was 275 L/min, and her best ever peak flow is 500 L/min. After a single salbutamol nebulizer, her peak flow improves to 455 L/min, and she feels much better.
Which of the following drug treatments should be administered next?