**Core Concept**
When a person transitions from a standing to a supine position, the body undergoes a series of physiological changes to accommodate the shift in blood distribution and pressure. This adaptation involves the autonomic nervous system, cardiovascular system, and the venous system.

**Why the Correct Answer is Right**
When a person changes from a standing to a supine position, gravity no longer pulls blood downward, and blood pools in the lower extremities. The decrease in venous return to the heart triggers a baroreceptor-mediated response, leading to a compensatory increase in sympathetic tone. This results in increased heart rate (tachycardia), cardiac contractility, and peripheral vascular resistance. The increased sympathetic tone also causes vasoconstriction in the splanchnic circulation, reducing blood flow to the gastrointestinal tract.

**Why Each Wrong Option is Incorrect**
**Option A:**
This is incorrect because the correct response to supine position is not bradycardia, but rather tachycardia due to the decrease in venous return and subsequent baroreceptor-mediated response.

**Option B:**
This is incorrect because the decrease in venous return to the heart does not lead to a decrease in cardiac output. Instead, the body compensates for the decrease in venous return by increasing heart rate and contractility.

**Option C:**
This is incorrect because the correct response to supine position is not vasodilation in the splanchnic circulation, but rather vasoconstriction, which reduces blood flow to the gastrointestinal tract.

**Option D:**
This is incorrect because the decrease in venous return to the heart does not lead to a decrease in peripheral vascular resistance. Instead, the body compensates for the decrease in venous return by increasing peripheral vascular resistance.

**Clinical Pearl / High-Yield Fact**
When patients with autonomic dysfunction, such as those with autonomic nervous system disorders, transition from a standing to a supine position, they may not exhibit the typical compensatory responses, leading to orthostatic intolerance and syncope.

**Correct Answer: C. Increased peripheral vascular resistance.**