Final answer:
The observation to report at once is a change in the ability to respond, which might indicate a stroke or TIA, requiring immediate medical attention. The FAST mnemonic can help identify such a situation rapidly.
Step-by-step explanation:
The observation that should be reported at once is when a person has a change in the ability to respond. This can be a sign of a serious neurological event, such as a stroke or TIA (Transient Ischemic Attack). The FAST mnemonic helps us recognize symptoms quickly: Face - problems moving face muscles; Arms - difficulty raising both arms; Speech - changes in speech or slurring; and Time to act fast if any symptoms are present. When delays in important neurological symptoms are not promptly addressed, the outcome can be compromised. Since recovery from a stroke is highly dependent on the speed of treatment, immediate reporting is critical to ensure rapid medical intervention.