Which symptom is considered a serious problem indicating immediate attention is needed in a child?

Diarrhea is considered a serious problem in children, especially when it is severe or persists over a certain period. It can lead to rapid dehydration, which is a critical condition that requires immediate medical attention. Children, particularly infants and young toddlers, are more vulnerable to dehydration because their bodies have a higher water turnover rate and they have less fluid reserve. Signs of dehydration, such as reduced urine output, dry mouth, lethargy, and increased thirst, can develop quickly and may escalate into a life-threatening situation if not treated promptly.

While headaches, skin rashes, and joint pain may also warrant medical evaluation, they do not typically present the same immediate risk of life-threatening dehydration that diarrhea does, making diarrhea a more urgent concern within the context of childhood illness management.