**Question:** A 24 years old female examined by an obstetrician in delivery room. Starting intravaginally, he injects lidocaine near the tip of the ischial spine. The nerve that is blocked by the drug is

A. Sciatic nerve
B. Sacral nerve
C. S2-S4 spinal nerves
D. Obturator nerve

**Core Concept:**

The ischial spine is a bony prominence on each side of the pelvis, located between the piriformis muscle and the iliacus muscle. The ischial spine is adjacent to the ischial tuberosity, which is the attachment site of the piriformis muscle. In obstetric analgesia, local anesthetic agents like lidocaine are used to provide regional anesthesia for pain relief during childbirth.

**Why the Correct Answer is Right:**

Lidocaine is a local anesthetic agent that blocks nerve conduction by inhibiting sodium channels in the nerve membrane, preventing the depolarization required for action potentials to travel down the nerve. When lidocaine is applied near the tip of the ischial spine, it can block the nerves that arise from the sacral plexus, particularly the S2-S4 spinal nerves.

**Why Each Wrong Option is Incorrect:**

A. Sciatic nerve (Option A) is not blocked by lidocaine near the ischial spine, as it arises from the lumbosacral plexus, which is separate from the sacral plexus where lidocaine is applied.

B. Sacral nerve (Option B) is a misnomer and should be referred to as sacral plexus, which is the target of lidocaine application for pain relief during childbirth.

C. Obturator nerve (Option C) also arises from the same plexus as the S2-S4 spinal nerves (sacral plexus) and is not specifically targeted by lidocaine near the ischial spine.

D. Obturator nerve (Option D) is another incorrect term, as it should be referred to as obturator nerve, which arises from the same plexus as S2-S4 spinal nerves and is not targeted by lidocaine near the ischial spine.

**Clinical Pearl:**

Targeting the sacral plexus allows for effective analgesia during childbirth, as the nerves that innervate the lower limb muscles and skin are derived from this plexus. By blocking these nerves, it helps to reduce pain during labor and delivery.