**Core Concept**
The cardiotoxicity of local anesthetics is primarily due to their ability to block sodium channels in the cardiac muscle, leading to a decrease in the cardiac action potential and ultimately, cardiac arrest. This effect is more pronounced in certain local anesthetics due to their high lipid solubility and ability to bind to cardiac tissue.

**Why the Correct Answer is Right**
Bupivacaine is the most cardiotoxic local anesthetic agent due to its high lipid solubility and its ability to bind to cardiac sodium channels, leading to a marked decrease in the cardiac action potential. This results in a significant prolongation of the QRS complex on the electrocardiogram and can progress to cardiac arrest. The mechanism of cardiotoxicity is thought to be due to the blockade of sodium channels, which prevents the normal influx of sodium ions during the action potential, leading to a decrease in the contractility of the heart muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** Lidocaine, while also a local anesthetic, has a lower lipid solubility and is less cardiotoxic than bupivacaine. It is often used in the treatment of cardiac arrhythmias due to its ability to block sodium channels in the heart.
**Option C:** Procaine is a local anesthetic with a low lipid solubility and is less cardiotoxic than bupivacaine. It is often used for minor surgical procedures and is less likely to cause significant cardiotoxicity.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the cardiotoxicity of local anesthetics is dose-dependent and can be mitigated by using lower doses and avoiding the intravascular injection of these agents. Additionally, the use of cardioprotective agents such as lidocaine and magnesium may help to reduce the risk of cardiotoxicity.

**Correct Answer:** B. Bupivacaine.