**Core Concept**
Ulnar nerve injury affects the **ulnar nerve**, which is responsible for innervating muscles and providing sensation to parts of the hand. The ulnar nerve controls movements and sensation in the **medial** part of the hand.

**Why the Correct Answer is Right**
The correct answer is related to the function of the ulnar nerve. Ulnar nerve injury leads to **sensory loss** of the little finger and ring finger, **weakness of the hypothenar muscles**, and **claw hand** due to the imbalance of muscle forces. However, **adduction of the thumb** is primarily controlled by the **adductor pollicis muscle**, which is innervated by the ulnar nerve, but the primary action of thumb adduction is performed by the **adductor pollicis** and assisted by other muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** is a possible consequence of ulnar nerve injury as it affects sensation.
**Option B:** is also a consequence as the hypothenar muscles are innervated by the ulnar nerve.
**Option C:** results from ulnar nerve injury due to muscle imbalance.

**Clinical Pearl / High-Yield Fact**
Ulnar nerve injury can lead to significant disability, including loss of fine motor function and sensation in the hand. The **Froment's sign** can be used to test for ulnar nerve injury, where the patient is asked to hold a piece of paper between the thumb and index finger.

**Correct Answer:** D. Adduction of thumb