**Core Concept**
Spinal anaesthesia involves injecting a local anaesthetic into the subarachnoid space, which can lead to complications due to the effects on the central nervous system and vasculature. The most common post-operative complication is related to the vasodilatory effects of the local anaesthetic.

**Why the Correct Answer is Right**
The most common post-operative complication of spinal anaesthesia is hypotension, which occurs due to the vasodilatory effects of the local anaesthetic. This leads to a decrease in systemic vascular resistance and a subsequent decrease in blood pressure. The local anaesthetic blocks the sympathetic nerves in the spinal cord, which normally constrict blood vessels and maintain blood pressure. As a result, there is a significant decrease in blood pressure, which can lead to inadequate perfusion of vital organs.

**Why Each Wrong Option is Incorrect**
**Option A:** Nerve damage is a potential complication of spinal anaesthesia, but it is not the most common post-operative complication. Nerve damage can occur due to direct trauma to the nerve roots during the procedure.
**Option B:** Infection is a potential complication of spinal anaesthesia, but it is relatively rare and not the most common post-operative complication. Infection can occur due to contamination of the needle or equipment.
**Option C:** Respiratory depression is a potential complication of spinal anaesthesia, but it is not the most common post-operative complication. Respiratory depression can occur due to the effects of the local anaesthetic on the respiratory centers in the brainstem.

**Clinical Pearl / High-Yield Fact**
To prevent hypotension during spinal anaesthesia, it is essential to use vasopressors, such as ephedrine, to maintain adequate blood pressure. Additionally, patients should be placed in a head-up position to reduce venous return and prevent further decreases in blood pressure.

**Correct Answer:** D. Hypotension