**Question:** Nerve involved in Guyon's canal syndrome is

A. Radial Nerve
B. Ulnar Nerve
C. Superficial Radial Nerve
D. Deep Radial Nerve

**Correct Answer:** B. Ulnar Nerve

**Core Concept:**
Guyon's canal is a fibro-osseous tunnel located at the anterolateral aspect of the distal part of the os pisiform, formed by the pisiform bone and the ulnar collateral ligament. The ulnar nerve, also known as the posterior interosseous nerve, runs through this canal and provides sensory innervation to the ulnar side of the hand and ulnar two-thirds of the forearm.

**Why the Correct Answer is Right:**
The ulnar nerve (B) is the nerve that runs through Guyon's canal, providing sensory innervation to the ulnar side of the hand and ulnar two-thirds of the forearm. The ulnar nerve is responsible for the symptoms associated with Guyon's canal syndrome.

**Why Each Wrong Option is Incorrect:**
A. Radial Nerve: The radial nerve is responsible for sensory innervation to the thumb, index, and middle fingers, and not affected in Guyon's canal syndrome.

C. Superficial Radial Nerve: This nerve is a branch of the radial nerve and is not involved in Guyon's canal syndrome, which affects the ulnar nerve.

D. Deep Radial Nerve: The deep radial nerve is a branch of the radial nerve, and Guyon's canal syndrome affects the ulnar nerve, not the deep radial nerve.

**Why Guyon's canal syndrome occurs:**
Guyon's canal syndrome is caused by compression or irritation of the ulnar nerve as it passes through the canal. Compression can occur due to various etiologies, such as:

1. Localized swelling, e.g., hematoma, tumor, or ganglion, which can compress the ulnar nerve as it passes through the canal.
2. Tight or constricting structures, such as the tendon of the flexor pollicis brevis muscle or the ulnar collateral ligament, which can lead to compression of the ulnar nerve.
3. Anatomical variations, such as a double Guyon's canal, which can lead to increased pressure on the ulnar nerve.

**Clinical Pearl:**
Guyon's canal syndrome is often referred to as the "hidden hand syndrome" due to its insidious onset and non-specific symptoms. Patients may present with symptoms such as pain, paresthesia, or weakness in the ulnar distribution, which can be easily confused with other hand and wrist conditions. A thorough clinical examination and knowledge of Guyon's canal anatomy is essential to make an accurate diagnosis and initiate appropriate treatment.