**Core Concept**
Lignocaine is a local anesthetic and antiarrhythmic drug belonging to the class of amino amides. For spinal anesthesia, lignocaine is used in combination with other drugs to produce a sensory and motor blockade.

**Why the Correct Answer is Right**
The dose of lignocaine for spinal anesthesia is typically around 5 mg. This is because lignocaine is a potent local anesthetic that can cause significant sensory and motor blockade at lower doses. The addition of other drugs like fentanyl or clonidine can enhance the analgesic effect while reducing the dose of lignocaine required. The optimal dose of lignocaine for spinal anesthesia is often considered to be between 4-6 mg, with a maximum dose of 7.5 mg recommended to minimize the risk of toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the dose of lignocaine for spinal anesthesia is typically lower than what is commonly used for epidural anesthesia.
* **Option B:** This option is incorrect because the dose of lignocaine for spinal anesthesia is not typically used for topical anesthesia, which requires much lower doses.
* **Option C:** This option is incorrect because the dose of lignocaine for spinal anesthesia is not typically used for intravenous regional anesthesia, which requires a specific dose and technique.

**Clinical Pearl / High-Yield Fact**
When using lignocaine for spinal anesthesia, it is essential to monitor the patient's vital signs closely and be prepared to manage any signs of toxicity, such as tinnitus, numbness, or seizures. The maximum dose of lignocaine for spinal anesthesia is 7.5 mg, and doses above this can increase the risk of toxicity.

**Correct Answer:** C. 5 mg.