**Question:** All of the following structures passes through the Guyon's canal, EXCEPT?

A. Pisiform bone
B. Ulnar nerve
C. Anterior interosseous nerve
D. Hypothenar muscles

**Core Concept:**
Guyon's canal is a tunnel in the hand that contains and protects the ulnar nerve and the deep branch of the ulnar artery and its accompanying veins. The structures that pass through this canal are essential for hand function and sensation. The canal is formed by the pisiform bone and the triquetral bone.

**Why the Correct Answer is Right:**
Option A, the pisiform bone, is part of the carpal bones and does not pass through Guyon's canal. It is located in the wrist, forming part of the pisiform-scaphoideus-triquetral complex.

**Why Each Wrong Option is Incorrect:**
Option B, the ulnar nerve, is indeed present in Guyon's canal and provides sensation to the ulnar side of the palm, ulnar two digits, and ulnar two digits' palmar aponeurosis.
Option C, the anterior interosseous nerve, is a branch of the brachial plexus and enters the forearm through the spiral groove, not Guyon's canal.
Option D, the hypothenar muscles, are located in the palm and not within Guyon's canal. The hypothenar muscles are part of the flexor pollicis brevis muscle group, which is innervated by the ulnar nerve in the forearm, not through Guyon's canal.

**Why Each Wrong Option is Incorrect:**
The anterior interosseous nerve (option C) is a branch of the brachial plexus and enters the forearm through the spiral groove, not Guyon's canal. This structure is essential for wrist and forearm movements.

**Clinical Pearl:**
The Guyon's canal syndrome occurs when the ulnar nerve is compressed within Guyon's canal, leading to symptoms such as pain, paresthesia, and muscle weakness in the ulnar two digits and ulnar two palmar aponeurosis. This condition is often caused by trauma, repetitive strain, or tumor compression. Diagnosis is clinical, and neurophysiological studies may be necessary for confirmation. Treatment may involve rest, splinting, and corticosteroid injections, or surgical decompression if symptoms persist.

**Why Each Wrong Option is Incorrect:**
Option D, hypothenar muscles (flexor pollicis brevis muscle group), are not present within Guyon's canal but are located in the palm, receiving innervation from the ulnar nerve in the forearm. This muscle group is responsible for abducting the thumb and flexing the little and ring fingers.