**Question:** Which is the nerve involved in case of ape thumb deformity?

**Core Concept:**
Appearance of an ape-like thumb deformity is primarily attributed to a congenital or acquired injury to the ulnar collateral ligament complex and the ulnar nerve. The ulnar nerve is responsible for providing motor and sensory innervation to the thumb and fingers.

**Why the Correct Answer is Right:**
The correct answer is **D.** The ulnar nerve (C8-T1) is the primary nerve involved in ape thumb deformity. This condition results from damage or dysfunction of the ulnar nerve, leading to the characteristic symptoms and deformity.

The ulnar nerve travels along the posterior aspect of the arm, passing through the Guyon canal at the wrist, and continues down the ulnar side of the wrist and hand. Injury to this nerve can lead to various symptoms and signs, including:

1. Hypoesthesia (loss of sensation) in the ulnar half of the palm, little finger, and half of the ring finger.
2. Hypotrophy (wasting) of the ulnar two digits (little finger and half of the ring finger).
3. Hypotrophy of the intrinsic muscles of the hand, causing the characteristic ape-like thumb deformity.

**Why Each Wrong Option is Incorrect:**

A. (Option A) The median nerve is responsible for the sensation and motor function of the thumb, not the ulnar nerve. Injury to the median nerve would not result in the ape thumb deformity seen in ulnar nerve injury.

B. (Option B) Although the radial nerve provides sensory innervation to the thumb, it does not cause the ape thumb deformity. The radial nerve's role in thumb function is different from the ulnar nerve's function in thumb motor function and sensation.

C. (Option C) The posterior interosseous nerve is responsible for the intrinsic muscles of the hand, but it does not cause the ape thumb deformity. Injury to the posterior interosseous nerve would result in a different pattern of hand weakness and deformity.

**Clinical Pearl:**

In clinical practice, it is essential to differentiate between the ulnar nerve and other nerves when assessing patients with hand and thumb dysfunction. A thorough physical examination, including manual muscle strength testing and sensory testing, is crucial for accurate diagnosis and appropriate management in cases of suspected peripheral nerve injuries.