Heart Anatomy and Conducting System

The sinoatrial (SA) node receives its arterial blood supply from which vessel in approximately 60% of the population?

Which of the following structures forms the anatomical boundary between the right and left atria on the internal surface of the heart?

A 72-year-old woman from Mumbai is admitted with acute inferior wall myocardial infarction. Coronary angiography reveals complete occlusion of the right coronary artery. On the cardiac monitor, she develops bradycardia (HR 35 bpm) with AV dissociation and a junctional escape rhythm (narrow QRS complexes at 40 bpm). The bradycardia is refractory to atropine. Which anatomical feature of the blood supply to the cardiac conduction system explains the failure of atropine to restore AV conduction?

A 58-year-old man from Delhi presents to the emergency department with acute-onset syncope during exertion. On examination, he is haemodynamically stable with a regular heart rate of 45 bpm. ECG shows a prolonged PR interval (240 ms) with progressive lengthening until a P wave is not followed by a QRS complex. After the dropped beat, the PR interval resets and the cycle repeats. Which anatomical structure of the cardiac conduction system is most likely affected?

Which electrophysiological property best distinguishes the sinoatrial (SA) node from the atrioventricular (AV) node in the cardiac conducting system?