**Core Concept**
High spinal anaesthesia refers to the accidental injection of local anaesthetic into the subarachnoid space above the level of the T4 vertebra. This results in sympathetic blockade of the thoracic and abdominal sympathetic nerves, leading to vasodilation and a subsequent decrease in blood pressure.

**Why the Correct Answer is Right**
The sympathetic blockade caused by high spinal anaesthesia leads to unopposed parasympathetic activity, resulting in bradycardia. The vasodilation of blood vessels in the thoracic and abdominal regions causes a decrease in peripheral resistance, leading to hypotension. This is because the sympathetic nervous system normally constricts blood vessels, while the parasympathetic nervous system causes vasodilation. In a high spinal anaesthesia, the sympathetic nervous system is blocked, allowing the parasympathetic nervous system to dominate, leading to bradycardia and hypotension.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypertension and tachycardia are not characteristic of high spinal anaesthesia. Sympathetic blockade would lead to vasodilation and decreased peripheral resistance, causing hypotension, not hypertension.

**Option B:** Hypertension is not a typical finding in high spinal anaesthesia. The sympathetic blockade would lead to vasodilation and decreased peripheral resistance, causing hypotension, not hypertension.

**Option C:** Tachycardia is not a typical finding in high spinal anaesthesia. The sympathetic blockade would lead to unopposed parasympathetic activity, causing bradycardia, not tachycardia.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor the patient's vital signs closely during spinal anaesthesia, as high spinal anaesthesia can lead to severe hypotension and bradycardia, potentially causing cardiac arrest. The use of vasopressors and atropine may be necessary to manage these complications.

**✓ Correct Answer: D. Hypotension, bradycardia**