**Core Concept**
Cardiac output (CO) is the volume of blood the heart pumps per minute, a critical determinant of tissue perfusion and oxygen delivery. CO is influenced by heart rate (HR), stroke volume (SV), and peripheral vascular resistance (PVR). Understanding the factors that affect CO is essential in managing patients with cardiovascular diseases.

**Why the Correct Answer is Right**
Cardiac output is decreased by factors that reduce heart rate, stroke volume, or increase peripheral vascular resistance. **Option C**, **Cold water immersion**, is a correct answer because it causes vasoconstriction, increasing PVR and reducing CO. This is due to the activation of the sympathetic nervous system, leading to the release of catecholamines like norepinephrine, which causes blood vessels to constrict.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a correct answer as it does not directly affect cardiac output. However, if you're thinking of a condition that increases CO, then this might be a distractor. But in this case, we are looking for a decrease in CO.
* **Option B:** This is not a correct answer as it does not directly affect cardiac output. However, if you're thinking of a condition that increases CO, then this might be a distractor. But in this case, we are looking for a decrease in CO.

**Clinical Pearl / High-Yield Fact**
Cold water immersion can cause a decrease in cardiac output by increasing peripheral vascular resistance, which can be detrimental in patients with underlying cardiovascular disease. This is an important consideration in the management of patients undergoing cold water immersion therapy for athletic performance or other purposes.

**Correct Answer:** C. Cold water immersion.