Final answer:
The most critical reaction requiring immediate intervention when administering morphine sulfate is respiratory depression, which can be life-threatening and may necessitate the use of naloxone to reverse its effects.
Step-by-step explanation:
The reaction that requires immediate intervention by the nurse when morphine sulfate is administered to Kristin is respiratory depression. Morphine, being an opiate, can lead to significant respiratory depression, which is a potentially life-threatening condition. While bradycardia and increased blood pressure are also concerns with certain cardiac medications and conditions, they do not universally require immediate intervention in the same way. Flushed skin is often a less serious side effect of morphine. It's important to note that naloxone is an opiate antagonist that can reverse respiratory depression caused by opioid overdose, highlighting the critical nature of this particular side effect.