A 68-year-old man with atrial fibrillation on digoxin presents for routine ECG. The tracing shows scooped ST depression in lateral leads, shortened QT interval, and flattened T waves. His serum potassium is 4.2 mEq/L, renal function is normal, and he reports no palpitations, nausea, or visual disturbances. The ECG findings marked **D** represent therapeutic digoxin effect. Which of the following clinical scenarios would most appropriately prompt immediate intervention and discontinuation of digoxin?