What is the most common cause of a seizure in a child under the age of 5 years?

In children under the age of 5 years, febrile seizures are the most common type of seizure. These seizures occur in response to fever, commonly associated with illnesses such as infections. The occurrence of seizures in this age group is often linked to a rapid increase in body temperature rather than the presence of a serious underlying condition.

Febrile seizures typically happen in the context of a temperature of 100.4°F (38°C) or higher, often during the first day of an illness. While experiencing a fever, especially from infections like viral syndromes, children's developing nervous systems may become more excitable, leading to seizure activity. Thus, fever stands out as the most prevalent trigger for seizures in younger children, making it the correct answer in this context.

The other causes listed, such as brain tumors, head trauma, and meningitis, are less common in this age group. While these conditions can indeed lead to seizures, their prevalence is significantly lower compared to the occurrence of febrile seizures resulting from fever.