**Core Concept**
The question is testing the knowledge of long-acting local anesthetics (LAs). Long-acting LAs are a class of medications that provide prolonged sensory and motor blockade, often used in procedures requiring prolonged anesthesia, such as orthopedic surgeries. These agents work by inhibiting the sodium channels in nerve fibers, thereby blocking the conduction of nerve impulses.

**Why the Correct Answer is Right**
The correct answer is **Bupivacaine**. Bupivacaine is a potent long-acting LA that belongs to the amino amide class. It acts by binding to the sodium channels in the nerve membrane, thereby preventing the depolarization of the nerve fiber and the transmission of pain signals. Bupivacaine has a long duration of action, typically lasting 6-12 hours, making it suitable for procedures requiring prolonged anesthesia.

**Why Each Wrong Option is Incorrect**
**Option A:** **Mepivacaine** is a short- to intermediate-acting LA, not a long-acting agent. While it has some advantages, such as a lower toxicity profile compared to bupivacaine, it is not as potent or long-lasting.

**Option C:** **Ropivacaine** is another long-acting LA, but it is not as potent as bupivacaine. Ropivacaine has a slightly lower toxicity profile and is often used in regional anesthesia, but it is not the longest-acting LA.

**Clinical Pearl / High-Yield Fact**
When using long-acting LAs like bupivacaine, it's essential to monitor the patient's vital signs closely, as these agents can cause cardiovascular and respiratory depression, especially in high doses.

**Correct Answer:** B. Bupivacaine