**Core Concept**
Carotid massage is a technique used to stimulate the vagus nerve and induce a reflex response in the body. The vagus nerve is responsible for controlling various involuntary functions, including heart rate and blood pressure.

**Why the Correct Answer is Right**
Carotid massage causes a reflex bradycardia, which is a decrease in heart rate. This occurs due to the stimulation of the vagus nerve, which releases acetylcholine, a neurotransmitter that acts on the sinoatrial (SA) node to slow down heart rate. The SA node is the natural pacemaker of the heart, responsible for generating electrical impulses that regulate heart contractions. The release of acetylcholine decreases the firing rate of the SA node, resulting in a slower heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** Carotid massage does not cause an increase in heart rate, as the vagus nerve stimulation has a parasympathetic effect, which is opposite to the sympathetic nervous system's effect.
**Option B:** Carotid massage does not cause a decrease in blood pressure, as the reflex bradycardia caused by vagus nerve stimulation would actually lead to an increase in blood pressure due to increased peripheral resistance.
**Option C:** Carotid massage does not cause a change in respiratory rate, as the primary effect of vagus nerve stimulation is on the cardiovascular system, not the respiratory system.

**Clinical Pearl / High-Yield Fact**
Carotid massage can be used as a diagnostic tool to assess the integrity of the vagus nerve and its reflex responses. It can also be used as a therapeutic technique to manage certain cardiovascular conditions, such as tachyarrhythmias.

**Correct Answer: C. Vagal response leading to reflex bradycardia.**