Final answer:
The most important assessment to be done first is to check the heart rate and rhythm of the elderly patient.
Step-by-step explanation:
The most important assessment to be done first is to check the heart rate and rhythm of the elderly patient. The patient's fall and subsequent memory loss could be a result of a head injury or other trauma, which could affect the cardiovascular system. Assessing the heart rate and rhythm can help determine if the patient is experiencing any cardiac issues as a result of the fall, such as a concussion, heart attack, or arrhythmia.