Which of the following would be considered a diagnostic sign?

A physical bruise observed by a rescuer is considered a diagnostic sign because it is an observable, objective finding that can indicate an injury or condition impacting the casualty's health. Diagnostic signs are physical manifestations that can help identify potential medical issues, making them essential for assessing a person's condition.

In contrast, the other options involve subjective descriptions or feelings. A description of pain from the casualty (the first option) relies on the patient's personal experience and perception, which can vary widely and is not readily observable by the rescuer. An emotional reaction from the patient (the third choice) also falls under subjective experience, reflecting their mental state rather than a physical condition. Lastly, a medical history taken from a family member may provide useful background information but does not constitute a physical sign or symptom directly observed in the patient, making it less reliable for immediate diagnosis.