## **Core Concept**
Bier's block, also known as intravenous regional anesthesia (IVRA), is a technique used to provide local anesthesia to a limb. It involves injecting a local anesthetic into a vein in the isolated limb. The **key principle** here is that the local anesthetic must be used in a concentration that is effective for anesthesia but not so high as to cause systemic toxicity when the tourniquet is released.

## **Why the Correct Answer is Right**
The correct answer, **0.5% Lignocaine (Lidocaine)**, is used for Bier's block because it provides effective anesthesia with a lower risk of systemic toxicity. Lignocaine (Lidocaine) is commonly used for IVRA due to its rapid onset and intermediate duration of action. A concentration of **0.5%** is typically recommended because it balances efficacy with safety; higher concentrations increase the risk of local anesthetic systemic toxicity (LAST) when the tourniquet is deflated.

## **Why Each Wrong Option is Incorrect**
- **Option A: 0.1% Lignocaine** - This concentration is likely too low to provide effective anesthesia for most procedures.
- **Option B: 2% Lignocaine** - This is a higher concentration than typically recommended for Bier's block and increases the risk of systemic toxicity upon tourniquet release.
- **Option D: 1% Lignocaine** - Although sometimes used in other types of local anesthesia, **1%** is higher than the usually recommended concentration for Bier's block.

## **Clinical Pearl / High-Yield Fact**
A critical aspect of performing a Bier's block is ensuring that the tourniquet remains inflated for a sufficient period after injection to prevent systemic spread of the local anesthetic. A **common recommendation** is to keep the tourniquet on for at least 20 minutes after injection to minimize the risk of local anesthetic systemic toxicity.

## **Correct Answer: C. 0.5% Lignocaine.**