A 48-year-old woman with dilated cardiomyopathy (EF 25%) presents with a single episode of syncope. Baseline 12-lead ECG shows sinus rhythm with frequent premature ventricular contractions (PVCs). She is on optimal medical therapy (ACE inhibitor, beta-blocker, aldosterone antagonist). Which investigation is most appropriate to risk-stratify her for sudden cardiac death and guide ICD therapy?