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Explanation:
The mean arterial blood pressure (MAP) is determined by cardiac output (CO) and systemic vascular resistance (SVR), following the relationship:
\[ \text{MAP} = \text{CO} \times \text{SVR} \]
Where:
- Cardiac Output (CO) is the volume of blood the heart pumps per minute, and it is the product of stroke volume (SV) and heart rate (HR).
- Systemic Vascular Resistance (SVR) is the resistance to blood flow in the systemic circulation.
Let's analyze each option:
A. **Increasing the afterload**: Afterload is the resistance the left ventricle has to overcome to eject blood. Increasing afterload generally increases SVR, which can increase MAP.
B. **Increasing sympathetic nervous system output**: The sympathetic nervous system increases heart rate and stroke volume (by increasing contractility) and causes vasoconstriction, all of which would increase MAP.
C. **Decreasing the diameter of the veins**: This leads to venoconstriction, which increases venous return, subsequently increasing stroke volume and cardiac output, thereby increasing MAP.
D. **Increasing the stroke volume**: An increase in stroke volume directly increases cardiac output, which increases MAP.
E. **Increasing parasympathetic nervous system output**: The parasympathetic nervous system primarily decreases heart rate and has minimal direct effect on stroke volume or vascular resistance. Increasing parasympathetic output would decrease heart rate and potentially reduce cardiac output, leading to a decrease in MAP.
Therefore, the correct answer is:
**E. Increasing parasympathetic nervous system output.**