Percentage of lignocaine used in spinal anaesthesia is :

**Core Concept**
Lignocaine is a local anesthetic commonly used in spinal anesthesia. It works by blocking the sodium channels in nerve fibers, preventing the transmission of pain signals to the brain. The percentage of lignocaine used in spinal anesthesia is critical in determining its effectiveness and toxicity.

**Why the Correct Answer is Right**
The correct answer is **0.5%**. This concentration is commonly used in spinal anesthesia because it provides a good balance between efficacy and toxicity. **0.5% lignocaine** is less likely to cause toxicity, such as central nervous system or cardiovascular effects, while still providing adequate analgesia. The addition of **dextrose** (usually 5%) to the lignocaine solution helps to reduce the pain associated with the injection and provides a mild vasodilatory effect.

**Why Each Wrong Option is Incorrect**
* **Option A:** **1% lignocaine** is too concentrated and increases the risk of toxicity, including central nervous system and cardiovascular effects.
* **Option B:** **0.2% lignocaine** is too dilute and may not provide adequate analgesia for spinal anesthesia.
* **Option C:** **1.5% lignocaine** is not a standard concentration used in spinal anesthesia and may increase the risk of toxicity.

**Clinical Pearl / High-Yield Fact**
When using lignocaine in spinal anesthesia, it's essential to remember that the addition of dextrose can affect the pH of the solution, which may impact the onset and duration of action.

**Correct Answer: D. 0.5% lignocaine**