This research will establish a mg/kg dose for a future RCT to determine the efficacy of high-dose oral montelukast for children with moderate and severe acute asthma exacerbations. Aim: Perform an adaptive, double-masked randomized controlled trial (RCT) of high-dose oral montelukast, with escalating mg/kg dose levels determined by PK-guided dose modeling, added to standard treatment versus standard treatment alone, in children with exacerbations that are moderate or severe after initial treatment with inhaled albuterol. Hypothesis 1: High-dose oral montelukast achieves peak plasma concentration (Cmax) \>1,700 ng/ml in \>86% of at least one of three sequential participant groups with escalating weight-based (milligram/kilogram or mg/kg) doses between groups. Hypothesis 2: Participants randomized to high-dose oral montelukast have a 2 point or greater improvement of the validated Acute Asthma Intensity Research Score (AAIRS) at 4 hours post-treatment in comparison with control group participants. Hypothesis 3: Among montelukast recipients, Cmax correlates with change of the AAIRS at 4 hours, after adjustment for pre-treatment exacerbation severity and systemic leukotriene stress measured using urinary leukotriene E4 (LTE4).