A 38-Year-Old Woman Is Evaluated for Sudden Onset of Shortness

A 38-year-old woman is evaluated for sudden onset of shortness of breath. On physical examination, the patient looks distressed. The blood pressure 130/80 mmHg, and the pulse is 88/min. ECG shows normal sinus rhythms, what is the site of the first electrical signal in the heart?

A) Left atrium near the cusps of the mitral valve.
B) Left atrium near the opening of pulmonary veins
C) Right atrium close to the opening of inferior vena cava
D) Right atrium close to the opening of superior vena cava