Which of the following is a possible cause of bradycardia?

Bradycardia, defined as a slower-than-normal heart rate (typically less than 60 beats per minute), can occur due to various physiological factors. One possible cause of bradycardia is the body's position, particularly when a person is lying down in a supine position. In this position, there can be increased vagal tone, which may lead to a decrease in heart rate. The vagus nerve, which plays a significant role in heart rate regulation, can become more active, especially in this position, resulting in bradycardia.

While dehydration can increase heart rate as the body attempts to maintain cardiac output, anxiety often causes an increase in heart rate due to the body's stress response. Overexertion may lead to tachycardia, not bradycardia, as the heart works harder to supply muscles with oxygen. Thus, the supine position is particularly noted for its association with a decrease in heart rate and is a definitive contributor to bradycardia.