**Core Concept**
The "gatekeeper of the heart" refers to the structure responsible for regulating blood flow and pressure into the heart. This concept is crucial in understanding cardiovascular physiology and pathophysiology.

**Why the Correct Answer is Right**
The correct answer is the sinoatrial (SA) node, also known as the natural pacemaker of the heart. The SA node generates electrical impulses that stimulate the atria to contract, initiating the cardiac cycle. This process ensures a consistent heart rate and coordinated contractions of the atria and ventricles. The SA node acts as a gatekeeper by controlling the influx of blood into the ventricles, regulating cardiac output, and maintaining systemic blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the atrioventricular (AV) node is involved in the electrical conduction system of the heart, but it does not act as the primary pacemaker or gatekeeper of the heart.
**Option B:** This option is incorrect because the bundle of His is a part of the electrical conduction system, transmitting impulses from the AV node to the ventricles, but it is not responsible for regulating blood flow into the heart.
**Option C:** This option is incorrect because the sinoatrial (SA) node is the correct answer, not the sinoventricular node, which is not a recognized structure in the heart.

**Clinical Pearl / High-Yield Fact**
The sinoatrial (SA) node is responsible for generating approximately 60-100 beats per minute in a healthy adult. This rate can increase or decrease in response to various physiological and pathological stimuli.

**Correct Answer: C. Sinoatrial (SA) node**