**Core Concept**
The sympathetic nervous system (SNS) is a branch of the autonomic nervous system that plays a crucial role in the body's "fight or flight" response. When activated, the SNS triggers a cascade of physiological changes to prepare the body for immediate action.

**Why the Correct Answer is Right**
When the SNS is activated, it releases the neurotransmitter norepinephrine (noradrenaline) from the nerve terminals, which binds to beta-adrenergic receptors on the heart, causing an increase in heart rate and cardiac output. This leads to increased blood flow to the muscles, liver, and other organs involved in the "fight or flight" response. Additionally, the SNS stimulates the release of epinephrine (adrenaline) from the adrenal medulla, which further contributes to the physiological changes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it doesn't accurately describe the physiological effects of SNS activation. While the parasympathetic nervous system does promote "rest and digest" functions, it's not directly related to the "fight or flight" response.
**Option B:** This option is incorrect because it's too vague and doesn't specify the physiological changes that occur during SNS activation. The SNS does affect various bodily functions, but this option doesn't provide enough detail.
**Option C:** This option is incorrect because it's a partial truth. While the SNS does stimulate the release of epinephrine, it's not the only hormone involved in the "fight or flight" response.

**Clinical Pearl / High-Yield Fact**
The SNS's "fight or flight" response is often triggered by stress, fear, or excitement, and can lead to increased blood pressure, heart rate, and respiration. It's essential to recognize the physiological changes that occur during SNS activation to provide appropriate medical care in emergency situations.

**Correct Answer: A. Increased heart rate and cardiac output.**