A. High fever can occur if the child develops an infection, which children with sickle cell disease are at increased risk for due to splenic dysfunction, but fever is not a hallmark of a sickle cell crisis. B. Bradycardia is not typically associated with sickle cell crisis; instead, children may exhibit tachycardia as a response to anemia, pain, or hypoxia. C. Constipation can occur with dehydration or certain medications, but it is not a primary symptom of sickle cell crisis. D. Pain is the hallmark symptom of sickle cell crisis, resulting from vaso-occlusion that blocks blood flow, causing tissue ischemia and severe discomfort.