A 28-year-old man with a known diagnosis of hypertrophic cardiomyopathy is undergoing risk stratification for sudden cardiac death (SCD). He reports one episode of lightheadedness without loss of consciousness during vigorous exercise 3 months ago. His father died suddenly at age 42. Echocardiography reveals asymmetric septal hypertrophy with a maximal interventricular septal thickness of 32 mm and a resting left ventricular outflow tract gradient of 45 mmHg. A 24-hour Holter monitor shows no episodes of nonsustained ventricular tachycardia. Which of the following findings is the most significant independent risk factor for SCD in this patient, indicating consideration for an implantable cardioverter-defibrillator (ICD)?