**Core Concept**
Lignocaine (lidocaine) is a local anesthetic and antiarrhythmic medication that works by blocking sodium channels in nerve fibers, preventing the initiation and transmission of nerve impulses. The maximum safe dose of lignocaine is limited by its potential to cause systemic toxicity, including central nervous system and cardiovascular effects.

**Why the Correct Answer is Right**
The maximum recommended dose of lignocaine for local anesthesia is 4.5 mg/kg body weight, with a maximum dose of 300 mg for a 70 kg adult. This dose is based on the risk of systemic toxicity, which can occur at higher doses due to the accumulation of lignocaine and its metabolites in the bloodstream. The therapeutic window for lignocaine is narrow, and doses above 300 mg can lead to symptoms such as dizziness, tremors, and seizures.

**Why Each Wrong Option is Incorrect**
**Option B:** 500 mg is significantly above the recommended maximum dose and increases the risk of systemic toxicity, including cardiac arrhythmias and respiratory depression.
**Option C:** 700 mg is even higher and can cause severe systemic toxicity, including seizures, coma, and cardiac arrest.
**Option D:** 1000 mg is far in excess of the recommended maximum dose and is likely to cause fatal systemic toxicity.

**Clinical Pearl / High-Yield Fact**
The "4 + 2" rule for lignocaine dosing is a useful mnemonic: a maximum dose of 4 mg/kg for the initial dose, with an additional 2 mg/kg for each subsequent dose, up to a maximum total dose of 300 mg.

**✓ Correct Answer: A. 300**