**Core Concept**
Ulnar nerve injury in the arm affects the motor and sensory functions of the hand, specifically involving the medial epicondyle of the humerus and the cubital tunnel. The ulnar nerve supplies the intrinsic muscles of the hand, specifically the interosseous muscles, the adductor pollicis, and the lumbrical muscles to the ring and little fingers.

**Why the Correct Answer is Right**
The ulnar nerve injury leads to weakness or paralysis of the intrinsic muscles of the hand, causing a positive Froment's sign, claw hand deformity, and sensory loss over the little and ring fingers. The ulnar nerve also supplies the medial two digits (ring and little fingers) with sensory innervation, which is responsible for the sensation of touch, pressure, and pain.

**Why Each Wrong Option is Incorrect**
**Option A:** Atrophy of the thenar muscles is seen in median nerve injury, not ulnar nerve injury.
**Option B:** Sensory loss over the ulnar two digits is a characteristic feature of ulnar nerve injury.
**Option C:** Claw hand deformity, also known as ulnar claw hand, is a classic sign of ulnar nerve injury, resulting from the paralysis of the interosseous and lumbrical muscles.
**Option D:** The Froment's sign is a positive sign of ulnar nerve injury, indicating the weakness or paralysis of the adductor pollicis muscle.

**Clinical Pearl / High-Yield Fact**
Ulnar nerve injury can be caused by compression or trauma to the ulnar nerve at the medial epicondyle of the humerus or within the cubital tunnel. A classic clinical sign of ulnar nerve injury is the positive Froment's sign, which is performed by asking the patient to hold a piece of paper between the thumb and index finger while the examiner tries to pull the paper away.

**Correct Answer: A. Atrophy of the thenar muscles is seen in median nerve injury, not ulnar nerve injury.**