**Question:** In spinal anaesthesia the drug is deposited between?

A. intervertebral disc spaces
B. vertebral arches
C. vertebral bodies
D. spinous processes

**Core Concept:** Spinal anaesthesia is a technique used to induce local anaesthesia in a specific region of the body by injecting a drug into the subarachnoid space, which surrounds the spinal cord and nerve roots. The subarachnoid space is located between the outer layer of the dura mater (arachnoid membrane) and the inner layer of the pia mater (endothelial cells lining the spinal cord).

**Why the Correct Answer is Right:** In spinal anaesthesia, the drug is deposited between the outer layer of the dura mater (arachnoid membrane) and the inner layer of the pia mater (endothelial cells lining the spinal cord). This placement ensures that the drug enters the subarachnoid space and blocks the conduction of sensory and motor nerves, causing anaesthesia in the lower limbs and, in some cases, the upper limbs as well.

**Why Each Wrong Option is Incorrect:**

A. Depositing the drug between intervertebral disc spaces would not achieve subarachnoid space entry, as the disc spaces are not part of the subarachnoid space.

B. Depositing the drug between the vertebral arches would not reach the subarachnoid space either. Vertebral arches are part of the vertebral column and do not have direct connection to the subarachnoid space.

C. Depositing the drug between the vertebral bodies would not be effective in achieving spinal anaesthesia. Vertebral bodies are part of the vertebral column and do not have direct access to the subarachnoid space.

D. Depositing the drug between the spinous processes would not allow the drug to enter the subarachnoid space. Spinous processes are part of the vertebral column and lack direct connection to the subarachnoid space.

**Clinical Pearl:** To effectively perform spinal anaesthesia, the anaesthesiologist should target the space between the arachnoid and pia mater, which is achieved by depositing the drug between the vertebral arachnoid membrane and the pia mater. This placement ensures the drug enters the subarachnoid space, where it will block the conduction of sensory and motor nerves, leading to the desired anaesthetic effect on the lower limbs and, in some cases, upper limbs as well.