**Core Concept:** The ulnar nerve is a major peripheral nerve that runs from the spinal cord, through the neck and upper arm, down the inside of the arm, and into the wrist and hand. It controls muscles and sensation in the little finger and half of the ring finger.

**Why the Correct Answer is Right:** Injuries to the ulnar nerve at the wrist, also known as ulnar nerve compression syndrome, are characterized by symptoms such as pain, numbness, and weakness in the distribution of the ulnar nerve. The correct answer (D) refers to a lesion involving the medial cord of the brachial plexus, which is a different location than the wrist, causing a different set of symptoms and signs.

**Why Each Wrong Option is Incorrect:**

A. Injury to the ulnar nerve at the elbow (entrapment neuropathy) is a different condition than a wrist injury, affecting a different part of the nerve and causing different symptoms.

B. Ulnar nerve compression at the wrist (Guillain-Mollaret's triangle) usually does not involve the axilla (armpit), causing different symptoms and signs.

C. Lesions involving the posterior interosseous nerve (a branch of the radial nerve) are distinct from ulnar nerve injuries at the wrist, causing different symptoms and signs.

**Clinical Pearl:** Guillain-Mollaret's triangle is a region where the ulnar nerve and the median nerve cross paths in the forearm, near the wrist, and can be a site for compression neuropathy.

**Correct Answer:** D. Lesion involving the medial cord of the brachial plexus (e.g., cubital tunnel syndrome)

**Explanation:** The ulnar nerve is primarily affected at the wrist (ulnar nerve compression syndrome), while the correct answer involves a lesion in the medial cord of the brachial plexus (e.g., cubital tunnel syndrome). This type of injury would lead to different symptoms and signs compared to an injury at the wrist, making it distinct and unrelated to the wrist injury.