## Core Concept
Cardiac output (CO) is the volume of blood the heart pumps per minute. It is calculated as the product of heart rate (HR) and stroke volume (SV): CO = HR × SV. Factors that increase cardiac output typically enhance either heart rate, stroke volume, or both.

## Why the Correct Answer is Right
The correct answer, , involves understanding that cardiac output is increased by factors such as an increase in heart rate (tachycardia), an increase in contractility of the heart (positive inotropes), or a decrease in systemic vascular resistance (afterload). This allows for a greater volume of blood to be pumped or an increase in the frequency of pumping.

## Why Each Wrong Option is Incorrect
- **Option A:** Increase in heart rate (tachycardia) directly increases cardiac output by increasing the number of contractions per minute, assuming stroke volume remains constant or also increases.
- **Option B:** Increase in contractility (positive inotropic agents) increases stroke volume, thereby increasing cardiac output.
- **Option D:** Decrease in systemic vascular resistance (afterload) can increase stroke volume according to the Frank-Starling mechanism, thereby increasing cardiac output.

## Why is Incorrect
- **Option C:** An increase in afterload (or systemic vascular resistance) typically decreases stroke volume because the heart must work harder to eject blood into a high-pressure system. This decrease in stroke volume usually cannot be compensated by an equal increase in heart rate, thus potentially decreasing cardiac output.

## Clinical Pearl / High-Yield Fact
A key clinical scenario to remember is that in heart failure, especially heart failure with reduced ejection fraction (HFrEF), increasing afterload (e.g., with vasopressors) can actually decrease cardiac output because the heart cannot efficiently pump against a high pressure.

## Correct Answer: C. Increase in afterload.