A 34-year-old man is brought to the Emergency Department after a brief collapse at work. The event was unwitnessed. He recalls feeling “unwell” beforehand but is unable to describe this clearly. Recovery was rapid, and he is now asymptomatic. He has no known cardiac disease and takes no regular medication.

Initial assessment shows normal observations. Cardiovascular and neurological examinations are unremarkable. A 12-lead ECG demonstrates sinus rhythm with no conduction abnormalities. Routine blood tests requested by triage are within reference ranges. You are asked to advise which single assessment is most likely to establish the underlying diagnosis in this presentation.

Which is the single best answer?