Final answer:
The most important observation for the nurse to report is the 40% body burn.
Step-by-step explanation:
The most important observation for the nurse to report in this situation is the fact that the client has burns over 40% of their body. This is a severe burn that requires specialized treatment and immediate medical attention. Burns of this extent can lead to complications such as infection, fluid loss, and organ dysfunction, so it is crucial for the nurse to inform the medical team about the extent of the burns.