A 28-year-old woman presents to the emergency department after a brief loss of consciousness while queuing at the supermarket. She reports feeling hot and nauseated before collapsing and recovered rapidly without confusion. Witnesses describe her as pale and sweaty but without tonic-clonic movements. Her ECG is normal, examination is unremarkable and baseline blood tests are normal. Which feature, if present, would most strongly indicate a high-risk cause of syncope requiring admission and urgent cardiology assessment?