You are answering a USMLE-style medical question. Question: A 3-year-old girl is brought to the emergency department by her mother 30 minutes after the sudden onset of shortness of breath, dizziness, abdominal pain, and urticaria. The symptoms began 5 minutes after she ate a peanut butter sandwich. Her respirations are 36/min and blood pressure is 84/50 mm Hg. There is stridor on auscultation of the chest. She is intubated and mechanical ventilation is begun. Intravenous fluid resuscitation and the appropriate pharmacotherapy are begun. The drug administered to this patient most likely has which of the following effects on cardiac pacemaker cells during an action potential? Options: A) Increased Ca2+ influx in phase 4 B) Increased Na+ influx in phase 3 C) Decreased Ca2+ influx in phase 0 D) Decreased K+ efflux during phase 3 " Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.