**Core Concept**
Spinal anaesthesia involves injecting a local anaesthetic into the subarachnoid space to block nerve impulses, resulting in temporary sensory and motor paralysis. The correct location for injection is crucial to achieve effective anaesthesia while minimizing complications.

**Why the Correct Answer is Right**
The subarachnoid space is located between the arachnoid mater and the pia mater, two of the three meningeal layers surrounding the spinal cord. Injecting the local anaesthetic here allows it to contact the cerebrospinal fluid, which then carries the anaesthetic to the spinal nerves, blocking their conduction. The proximity to the spinal nerves ensures rapid onset of anaesthesia.

**Why Each Wrong Option is Incorrect**
**Option A:** The epineurium is the outermost layer of the nerve sheath, enveloping the individual nerve fascicles. Injecting into this space would be ineffective, as the local anaesthetic would not reach the nerve fibers.
**Option B:** The subdural space lies between the dura mater and the arachnoid mater. Injecting here would lead to a less effective anaesthesia, as the local anaesthetic would not be in direct contact with the cerebrospinal fluid.
**Option C:** The epidural space is located outside the dura mater, between it and the vertebral canal. While epidural anaesthesia is a different technique, injecting into this space would not provide the same level of spinal nerve blockade as subarachnoid injection.

**Clinical Pearl / High-Yield Fact**
To identify the correct location for spinal anaesthesia, it is essential to perform a midline approach, ensuring the needle is centered between the spinous processes. This reduces the risk of accidental dural puncture or epidural injection.

**Correct Answer:** L3-L4 interspace.