**Core Concept**
The Bezold-Jarisch reflex is a complex physiological response to ventricular distension, characterized by a decrease in heart rate and blood pressure. This reflex is mediated by the activation of mechanoreceptors in the ventricular wall, which triggers a series of neural signals that ultimately lead to a reduction in sympathetic tone and an increase in parasympathetic tone.

**Why the Correct Answer is Right**
Ventricular distension activates mechanoreceptors in the ventricular wall, specifically the stretch-sensitive receptors, which are sensitive to changes in ventricular volume and pressure. The activation of these receptors triggers a neural signal that travels to the nucleus ambiguus in the medulla oblongata, where it stimulates the release of acetylcholine, a neurotransmitter that increases parasympathetic tone and decreases sympathetic tone. This results in a decrease in heart rate and blood pressure. The Bezold-Jarisch reflex is an important mechanism for regulating cardiovascular function and preventing excessive increases in blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Atrial overload, while it can lead to increased pressure and stretch in the atria, does not directly activate the Bezold-Jarisch reflex, which is specifically mediated by ventricular distension.
**Option B:** Myocardial infarction can lead to changes in ventricular function and distension, but it is not the direct stimulus for the Bezold-Jarisch reflex.
**Option D:** Isotonic exercise, which involves muscle contraction without significant changes in blood pressure, does not directly activate the Bezold-Jarisch reflex.

**Clinical Pearl / High-Yield Fact**
The Bezold-Jarisch reflex is an important mechanism for regulating cardiovascular function, particularly in situations where there is excessive ventricular distension, such as in cardiac tamponade or severe heart failure. Understanding this reflex is crucial for managing patients with cardiovascular disease.

✓ Correct Answer: C. Ventricular distension.