Final answer:
The nurse is most concerned about the side effect of respiratory depression when administering intravenous hydromorphone, an opioid analgesic, to a 12-year-old patient with multiple injuries.
Step-by-step explanation:
The nurse administering intravenous hydromorphone to a 12-year-old boy who was involved in a motor vehicle accident (MVA) is most concerned about the side effect of respiratory depression. Hydromorphone, an opioid analgesic, can cause various side effects, including respiratory depression, particularly at higher doses. The risk of respiratory depression is the most immediate and potentially life-threatening side effect when administering opioids. Opioids suppress the respiratory center in the brain, leading to slower and potentially inadequate breathing. The nurse must closely monitor the patient's respiratory status and be prepared to provide interventions if needed.