After ensuring the stability of airway, breathing, and circulation, what is the most appropriate first-line treatment in a teenage boy with a severe asthma attack (already on salbutamol and beclomethasone)? Controlled oxygen therapy with a FiO2 of 24% Salbutamol 500 micrograms nebulized Terbutaline 10 mg nebulized Hydrocortisone 200 mg intravenously Continuous positive airway pressure ventilation