**Core Concept:** Spinal anaesthesia is a regional anaesthesia technique where a local anaesthetic drug is injected into the subarachnoid space, which surrounds the spinal cord and nerve roots. This results in the blockade of sensory and motor function in a specific region of the body, effectively rendering the patient insensate to pain and immobilizing the affected limbs.

**Why the Correct Answer is Right:** The correct answer, "A" (between the subarachnoid space and the spinal cord), represents the correct location for injecting the local anaesthetic drug in spinal anaesthesia. The subarachnoid space is a small space between the outer covering of the spinal cord (dura mater) and the inner covering of the nerves (pia mater). When the local anaesthetic is injected into this space, it spreads and blocks the conduction of the nerve impulses, causing the desired effect of sensory and motor blockage in a specific region of the body.

**Why Each Wrong Option is Incorrect:**

B. Injection in the epidural space is incorrect because this space is located between the outer covering of the spinal cord (dura mater) and the spinal cord itself. Injecting the drug here would not block the conduction of nerve impulses effectively.

C. Injection between the vertebrae is incorrect because this option is unrelated to the proper injection site for spinal anaesthesia. Vertebrae are the bony structures that make up the spine, and injecting the drug between them will not achieve the desired effect.

D. Injection within the spinal cord is incorrect, as injecting the drug within the spinal cord would cause a severe and potentially life-threatening complication known as spinal cord injury or ischemia. This site is not suitable for administering local anaesthesia.

**Clinical Pearl:** The correct injection site in spinal anaesthesia is crucial to avoid complications and ensure effective blockage. The drug should be injected in the subarachnoid space, between the subarachnoid space and the spinal cord, to achieve the desired effect of sensory and motor blockage in a specific region of the body.