**Core Concept**
The question is testing the understanding of the systemic effects of local anesthetics (LAs) toxicity, specifically with regards to the dose-dependent effects of LA overdose. Local anesthetics work by blocking sodium channels in nerve cells, preventing the initiation and transmission of nerve impulses.

**Why the Correct Answer is Right**
Local anesthetic systemic toxicity (LAST) occurs when the dose of LA exceeds the amount needed to block nerve conduction in a specific area. At higher doses, LAs can cause central nervous system (CNS) effects, including:
- **Option A:** Dizziness and sedation are not specific or sensitive markers for local anesthetic overdose. These symptoms can occur at therapeutic doses and are not a reliable sign of toxicity.
- **Option B:** Seizure is a specific and sensitive marker for local anesthetic overdose. As the LA concentration increases, it can cause CNS excitation, leading to seizure activity. This is a well-documented effect of LA toxicity.
- **Option C:** Muscle weakness is a possible effect of LA toxicity, but it is not as specific or sensitive as seizure activity. Muscle weakness can result from LA-induced blockade of nerve conduction, but it is not a reliable marker for overdose.
- **Option D:** Respiratory depression is a serious consequence of LA toxicity, but it is not the most specific or sensitive marker for overdose. While respiratory depression can occur, it is often a late sign and may not be the first manifestation of toxicity.

**Clinical Pearl / High-Yield Fact**
LAST can be prevented or minimized by careful dosing, monitoring, and avoiding the use of LAs in patients with underlying medical conditions. Always use the minimum effective dose and be aware of the patient's vital signs and neurological status.

**Correct Answer:** B. Seizure is a specific and sensitive marker for local anesthetic systemic toxicity (LAST).