This presentation is most consistent with acute bronchiolitis, which classically affects infants under 2 years (peak 2–6 months) and begins with upper respiratory symptoms (coryza, cough, low-grade fever) followed by increased work of breathing, tachypnea, and wheeze. The most common causative agent is respiratory syncytial virus (RSV), making it the best answer. Parainfluenza virus is more strongly associated with croup (barking cough, inspiratory stridor, hoarseness). Chlamydia trachomatis pneumonia typically occurs in young infants with a staccato cough, afebrile course, and may be associated with conjunctivitis rather than acute wheezing after coryza. Human metapneumovirus can cause bronchiolitis-like illness but is less common than RSV. Influenza A often produces higher fever and systemic symptoms and is not the leading cause of bronchiolitis in this age group. MCCQE objectives emphasize recognizing bronchiolitis clinically and identifying RSV as the predominant pathogen in infants with this classic progression of symptoms.