A 34-year-old man is brought to the Emergency Department after a brief transient loss of consciousness at work. The event was unwitnessed. Before collapsing, he recalls feeling light-headed and generally unwell for a few moments. He fell to the floor but regained consciousness quickly without intervention.

There was no prolonged confusion, no tongue biting, no witnessed limb jerking, and no urinary incontinence. He is now fully alert and feels back to normal.

He has no known cardiac disease, no previous seizures, and takes no regular medication.

On arrival, his observations are within normal limits. A single blood pressure reading is normal. Cardiovascular and neurological examinations are unremarkable. A 12-lead electrocardiogram has been requested and is pending. Routine blood tests sent at triage are within reference ranges.

According to NICE CG109 on transient loss of consciousness, which single assessment is most likely to establish the underlying diagnosis in this presentation?