Answer: Substernal pain
Explanation:Oxygen toxicity is a condition that can occur when a person receives high levels of oxygen for an extended period. In this case, the nurse is assessing a preschooler with cystic fibrosis who has been receiving oxygen therapy for 36 hours. One of the indications that the child has developed oxygen toxicity is substernal pain. This pain is a result of the inflammation of lung tissue caused by high oxygen levels.