**Core Concept**
When a person stands suddenly from a lying down posture, the body experiences a rapid shift in blood distribution due to gravity, leading to a decrease in venous return to the heart. This triggers a series of physiological responses to maintain blood pressure and ensure adequate cerebral perfusion.

**Why the Correct Answer is Right**
In response to standing, the baroreceptors in the carotid sinus and aortic arch detect the decrease in blood pressure and send signals to the brainstem. The brain interprets this as a decrease in blood volume and activates the sympathetic nervous system to counteract the effect. One of the key responses is the increased tone of capacitance vessels, such as veins, which helps to redirect blood flow to the heart and brain. This is mediated by the release of noradrenaline, which acts on alpha-adrenergic receptors to cause vasoconstriction of capacitance vessels.

**Why Each Wrong Option is Incorrect**
**Option B:** The IX cranial nerve, also known as the glossopharyngeal nerve, is primarily responsible for swallowing and taste sensation, and is not directly involved in the physiological response to standing.

**Option C:** In fact, the heart rate typically increases in response to standing due to the activation of the sympathetic nervous system, which increases the firing rate of the sinoatrial node.

**Option D:** Only option A is correct, so option D is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the body's response to standing involves a complex interplay of autonomic nervous system responses, including sympathetic activation and parasympathetic withdrawal. Understanding these mechanisms is crucial for managing patients with orthostatic hypotension or other conditions that affect blood pressure regulation.

**✓ Correct Answer: A. Increased tone of capacitance vessels.**