In the human cardiac conduction system, which structure possesses the highest intrinsic pacemaker discharge rate and serves as the primary cardiac pacemaker?
Answer and explanation
Correct answer: C. Sinoatrial (SA) node
The sinoatrial (SA) node, situated in the upper wall of the right atrium near the entrance of the superior vena cava, has the fastest spontaneous diastolic depolarization rate (typically 60–100 discharges per minute in an adult at rest) driven by hyperpolarization-activated cyclic nucleotide-gated 'funny' channels (If). It therefore depolarizes first and overrides the slower secondary pacemakers (AV node: 40–60 bpm, Purkinje fibers: 20–40 bpm).
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