**Core Concept**
Lignocaine (lidocaine) is a local anesthetic commonly used in spinal anesthesia to block sensory and motor nerve signals in the spinal cord. The optimal dose of lignocaine for spinal anesthesia is crucial to ensure effective analgesia without causing excessive toxicity.

**Why the Correct Answer is Right**
The recommended dose of lignocaine for spinal anesthesia is typically between 50-100 mg. This dose range is sufficient to provide effective sensory and motor blockade without causing significant toxicity. The exact dose may vary depending on individual patient factors, such as weight and the desired level of blockade. **Lignocaine works by blocking voltage-gated sodium channels in nerve fibers, preventing the initiation and transmission of nerve impulses.**

**Why Each Wrong Option is Incorrect**
**Option A:** 200 mg - This dose is excessively high and may cause significant toxicity, including central nervous system (CNS) effects such as seizures and cardiovascular instability.

**Option B:** 20 mg - This dose is too low and may not provide effective sensory or motor blockade, leading to inadequate pain relief.

**Option C:** 150 mg - While not excessively toxic, this dose may cause more pronounced CNS effects, such as dizziness and nausea, and may not provide the desired level of blockade.

**Clinical Pearl / High-Yield Fact**
When administering lignocaine for spinal anesthesia, it is essential to use a dose that is adjusted to the patient's weight and the desired level of blockade to minimize the risk of toxicity.

**Correct Answer: C. 75 mg**