You are answering a USMLE-style medical question. Question: A 55-year-old male presents to the emergency department with dyspnea and palpitations. He does not smoke cigarettes or consume alcohol, and past medical history is insignificant. Blood pressure is 115/75 mmHG and heart rate is 125/min. Heart rhythm is irregularly irregular. After initial treatment with IV digoxin the patient's heart rate drops to 85/min and remains irregular. Which of the following best explains the effect of digoxin in this patient? Options: A) Blockade of beta-adrenergic receptors B) Calcium channel blockade C) Vagal stimulation D) Increased ventricular contractility Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.