Croup
Viral laryngotracheobronchitis causes subglottic mucosal swelling at the narrowest, fixed-diameter part of a young child's airway, producing a barking cough and stridor whose severity is read directly off the degree of narrowing.
First principles
The swelling happens at the one point in the airway that cannot expand
Croup is caused by parainfluenza virus (usually) infecting the larynx, trachea and bronchi. Inflammatory oedema collects in the subglottic region, which is the only part of the paediatric airway completely encircled by rigid cartilage: the cricoid ring. Everywhere else the airway can distend slightly to accommodate swelling; here it cannot, so even a small amount of mucosal oedema produces a disproportionate reduction in cross-sectional area. This single anatomical fact explains why a mild coryzal illness in a toddler can suddenly produce dramatic airway noise.
Educational content pending clinical review. Not medical advice.