The correct answer is Bupivacaine.

- Among the commonly used local anesthetics, bupivacaine is well-known for its higher risk of cardiotoxicity when inadvertently administered intravenously or absorbed in excessive amounts.
- This is primarily due to its high lipid solubility and strong binding affinity to cardiac sodium channels, which results in prolonged sodium channel blockade and impaired cardiac conduction and contractility.
- Clinically, this can manifest as ventricular arrhythmias, severe hypotension, and even cardiac arrest.

In contrast, other local anesthetics like lidocaine and mepivacaine have a comparatively safer cardiac profile. Lidocaine, although it has some cardiac effects, is often used as an antiarrhythmic agent and is less likely to cause severe cardiotoxicity at accidental intravenous doses. Mepivacaine and prilocaine also have lower cardiotoxicity risk relative to bupivacaine.

Key points:
- Bupivacaine has a high affinity for cardiac sodium channels, leading to more profound and prolonged sodium channel blockade.
- This causes severe cardiac arrhythmias and cardiotoxicity in the event of accidental intravascular injection.
- Lidocaine and other local anesthetics like mepivacaine and prilocaine have a comparatively lower risk of cardiotoxicity.
- Caution and proper aspiration before injection are essential to minimize intravascular administration risks with bupivacaine.

Reference: Harold Ellis, Clinical Anesthesia, Volume 1, Chapter 15 - Local Anesthetics and Their Systemic Toxicity, Page 265-270.