A 23-year-old woman comes to the asthma clinic for a follow-up. She says she still has night-time awakenings and has to use her reliever medication more often than not. She was diagnosed with asthma 2 years ago. Her primary care physician commenced her on a low-dose as-needed inhaled corticosteroid (ICS) and long-acting beta 2 agonist (LABA) formoterol/budesonide combination inhaler. Despite her being regular and adherent, she has required modifications in her therapy, reaching high dose ICS plus LABA as maintenance and reliever therapy (MART). She has had 3 exacerbations requiring hospitalization, the most recent being an year ago. She was recently stepped down from a course of oral steroids. She does not smoke or drink alcohol, does not have any pets, and her records show no food allergies. Her temperature is 37.0 ˚C (98.6 ˚F), pulse is 88/min, respirations are 17/min and blood pressure is 110/68 mm Hg. Oxygen saturation is 97% on air. She had a skin prick test that was negative. Her full blood count was essentially normal, except for blood eosinophil levels of 370 cells/µL (normal: 40-450). The decision is made to add another medication to her treatment plan. Which of the following is the most appropriate medication to commence?