**Core Concept**
Local anaesthetics can be classified based on their cardiotoxic potential, with some being more prone to causing cardiac arrhythmias and other systemic complications. The cardiotoxicity of local anaesthetics is related to their ability to block sodium channels in the heart, leading to changes in ion channel function and ultimately cardiac arrest.

**Why the Correct Answer is Right**
Bupivacaine is a potent, long-acting local anaesthetic known for its high cardiotoxic potential. It has a high lipid solubility, which allows it to easily penetrate cardiac membranes and bind to sodium channels, leading to a high risk of cardiac arrhythmias and arrest. Bupivacaine's cardiotoxic effects are thought to be due to its ability to block sodium channels in a voltage-dependent manner, leading to a prolonged QRS interval and ultimately cardiac arrest.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lidocaine is actually used to treat cardiac arrhythmias due to its ability to block sodium channels and reduce automaticity in the heart. It is not cardiotoxic.
* **Option C:** Mepivacaine is a local anaesthetic with a lower cardiotoxic potential compared to bupivacaine. While it can still cause systemic toxicity, it is less likely to cause cardiac arrhythmias.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the order of local anaesthetic cardiotoxicity is: bupivacaine > etidocaine > mepivacaine > lignocaine. This knowledge can help you quickly identify the most cardiotoxic local anaesthetic in a given situation.

**Correct Answer:** B. Bupivacaine