An 82-year-old woman has had increasing fatigue for the past 2 years. During this time, she has experiencedparoxysmal dizziness and syncope. On physical examination, she is afebrile. Her pulse is 44/min, respirations are 16/min, and blood pressure is 100/65 mm Hg. On auscultation, the lungs are clear, and no murmurs are heard. An echocardiogram shows a normal-sized heart with normal valve motion and estimated ejection fraction of 50% After being treated with digoxin, the heart rate slows and becomes irregular. An abnormality involving which of the following is most likely to be present in this patient? (A) Atrioventricular node (B) Bundle of His (C) Left bundle branch (D) Parasympathetic ganglion (E) Right bundle branch (F) Sinoatrial node (G) Sympathetic ganglion