A 4-year-old girl experiences spontaneous nosebleeds. What is the most likely etiology of her nosebleeds?

Spontaneous nosebleeds in young children are commonly attributed to dry air. The nasal mucosa can become dry and irritated in conditions where humidity levels are low, leading to crusting and subsequent bleeding when the child or their environment disturbs the nasal passages. Children are often more susceptible due to their smaller nasal passages and the frequent occurrence of upper respiratory infections, which can exacerbate dryness.

While hypertension, pyogenic granuloma, and von Willebrand disease could potentially cause nosebleeds, they are less likely in this context. Hypertension is relatively uncommon in young children and usually manifests with additional systemic signs. Pyogenic granulomas are benign vascular growths that could cause nasal bleeding, but they are not typically seen as a primary cause of spontaneous nosebleeds in otherwise healthy children. Von Willebrand disease, a genetic bleeding disorder, would present with additional signs of bleeding, such as easy bruising or prolonged bleeding from minor injuries, rather than isolated epistaxis in a healthy child. Thus, dry air stands out as the most probable cause in this scenario.