**Core Concept:** Spinal anaesthesia is a regional anesthesia technique that involves injecting a local anesthetic drug into the subarachnoid space, which results in the blockage of sensory nerves and motor nerves in the lower limbs. The blockade starts from the higher segments and descends towards the lower segments.

**Why the Correct Answer is Right:** In spinal anaesthesia, the local anesthetic drug (commonly bupivacaine or lidocaine) is injected into the subarachnoid space, which is located between the outer layer of the meninges (dura mater) and the inner layer (pia mater). The drug diffuses into the cerebrospinal fluid (CSF) and travels along the ventral (sensory) and dorsal (motor) rootlets of the spinal cord. As the spinal cord is divided into different segments, the blockade occurs in a segmental manner.

The correct answer, **C. Cervical nerves (C1-C8)**, refers to the cervical spinal nerves that are blocked first in spinal anesthesia. These nerves are responsible for sensory and motor functions in the upper extremities and face.

**Why Each Wrong Option is Incorrect:**

A. **Dorsal root ganglia (DRG):** These are not blocked in spinal anesthesia; instead, the local anesthetic drug blocks the sensory and motor nerves within the spinal cord segments (C1-C8) that the DRG innervates.

B. **Lumbar nerves (L1-L5):** While spinal anesthesia does involve blocking lower limb nerves, lumbar nerves are not blocked first, as they are lower than the cervical nerves (C1-C8).

D. **S2-S4:** Motor nerves in the lower limbs are blocked in spinal anesthesia, but these nerves (S2-S4) are lower than the cervical nerves (C1-C8), so they are not blocked first.

**Clinical Pearl:** In spinal anesthesia, the success of the procedure depends on the depth and volume of the injected local anesthetic drug. Ideally, a small volume (10-15 mL) of hyperbaric (having a density greater than cerebrospinal fluid) solution is usually administered at the L3-L4 or L4-L5 interspace to produce effective blockade of lower limb sensory and motor nerves.

**Why the Correct Answer is Right:**

Cervical nerves (C1-C8) are blocked first in spinal anesthesia because they innervate the upper extremities and face, which are important for maintaining vital functions like breathing, circulation, and consciousness. By blocking these nerves first, the anesthesia can be safely administered without affecting the patient's vital functions. Additionally, once these nerves are blocked, the local anesthetic drug can travel distally to block the lower limb nerves (S2-S4) and thoracic nerves (T1-T12), thereby achieving a successful spinal anesthesia.

**Why Each Wrong Option is Incorrect:**

A. Blocking lumbar nerves (L1-L5) is incorrect because these nerves innervate lower limbs, which are blocked later in the procedure.