**Core Concept**
The vagus nerve plays a crucial role in regulating heart rate through its parasympathetic influence. The vagus nerve uses acetylcholine as its neurotransmitter to stimulate the sinoatrial (SA) node, which is the heart's natural pacemaker.

**Why the Correct Answer is Right**
When the vagus nerve is stimulated, it releases acetylcholine onto the muscarinic receptors in the SA node. This binding of acetylcholine to muscarinic receptors decreases the firing rate of the SA node, leading to a decrease in heart rate. This phenomenon is known as negative chronotropy. The SA node's decreased firing rate results in a slower heart rate, which is a classic example of the parasympathetic nervous system's effect on the heart.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because vagal stimulation does not cause an increase in heart rate. The parasympathetic nervous system, which includes the vagus nerve, has a negative inotropic and chronotropic effect on the heart.
* **Option B:** This option is incorrect because the vagus nerve does not stimulate the release of epinephrine from the adrenal glands. Epinephrine is released by the sympathetic nervous system, which has a positive inotropic and chronotropic effect on the heart.
* **Option C:** This option is incorrect because the vagus nerve does not decrease the refractory period of the heart. The refractory period is the time during which the heart muscle is unable to contract again after a heartbeat. The parasympathetic nervous system does not have a direct effect on the refractory period.

**Clinical Pearl / High-Yield Fact**
The parasympathetic nervous system's influence on the heart is mediated by the vagus nerve, which releases acetylcholine to decrease heart rate. This is an important concept to remember when considering the effects of various medications and conditions on the heart.

**Correct Answer:** D. Decreased heart rate.