**Core Concept**
Hypotension following acute spinal injury results from disruption of the spinal cord's control over autonomic functions, particularly sympathetic outflow. The sympathetic nervous system regulates vascular tone and heart rate, and its loss leads to unopposed vasodilation and reduced cardiac output.

**Why the Correct Answer is Right**
In acute spinal injury, especially at or above the T6 level, the spinal cord's ability to maintain sympathetic tone is compromised. This results in **loss of sympathetic vasoconstrictor tone**, leading to widespread vasodilation, decreased systemic vascular resistance, and subsequent hypotension. The mechanism involves interruption of descending sympathetic pathways from the thoracolumbar region, which normally maintain arterial pressure. This is a classic feature of spinal shock and autonomic dysreflexia, though hypotension here is due to loss of sympathetic tone rather than reflex spasm.

**Why Each Wrong Option is Incorrect**
Option B: Parasympathetic tone primarily regulates gastrointestinal and genital functions, not blood pressure. Its loss does not cause hypotension.
Option C: Orthostatic hypotension occurs due to postural changes and is not the primary cause in spinal trauma.
Option D: Vasovagal attacks are reflexive, episodic, and typically triggered by stress or standing, not spinal injury.

**Clinical Pearl / High-Yield Fact**
In spinal cord injuries, **hypotension due to loss of sympathetic tone** is most common in injuries at or above T6, and it is a key feature of spinal shock. Always assess for hypotension and shock in spinal trauma patients—early recognition is vital for fluid resuscitation.

✓ Correct Answer: A. Loss of sympathetic tone