Which of the following is a key sign that a patient is experiencing syncope?

When considering the signs of syncope, the key characteristic is the rapid onset of unresponsiveness after losing consciousness. Syncope, commonly known as fainting, is usually a result of a temporary decrease in blood flow to the brain. The immediate unresponsiveness indicates a quick drop in consciousness, which aligns directly with how syncope typically manifests in patients.

In contrast, being overheated and sweaty can point to different medical issues such as heat exhaustion, rather than syncope specifically. Losing bladder control is often associated with other conditions, such as seizures or when someone is in a state of unconsciousness for a prolonged period rather than the brief nature of syncope. Lastly, prolonged shaking for several minutes is more characteristic of a seizure than syncope, where the patient typically does not have muscle contractions or movements. Thus, the immediate unresponsiveness following loss of consciousness clearly defines the key sign of a syncope event.