**Core Concept**
The ulnar nerve is a branch of the brachial plexus that supplies the intrinsic muscles of the hand, specifically the hypothenar muscles, and provides sensory innervation to the little and ring fingers. The nerve divides into superficial and deep branches at the level of the pisiform bone.

**Why the Correct Answer is Right**
The patient's symptoms of numbness of the little and ring fingers and atrophy of the hypothenar muscles suggest damage to the ulnar nerve before it divides into its superficial and deep branches. This is because the ulnar nerve supplies the hypothenar muscles and the palmar cutaneous branch of the ulnar nerve does not supply the little and ring fingers. The deep branch of the ulnar nerve primarily supplies the intrinsic muscles of the hand, excluding the hypothenar muscles. The posterior cord of the brachial plexus primarily gives rise to the axillary and radial nerves, and does not directly innervate the ulnar nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** The palmar cutaneous branch of the ulnar nerve primarily supplies the skin of the hypothenar eminence and does not innervate the intrinsic muscles of the hand.
**Option B:** The deep branch of the ulnar nerve primarily supplies the intrinsic muscles of the hand, excluding the hypothenar muscles, and does not cause atrophy of the hypothenar muscles.
**Option D:** The posterior cord of the brachial plexus primarily gives rise to the axillary and radial nerves, and does not directly innervate the ulnar nerve.

**Clinical Pearl / High-Yield Fact**
The ulnar nerve is commonly damaged at the elbow, particularly at the cubital tunnel, where it passes through a fibro-osseous canal. This is a classic site for compression neuropathy, often resulting in numbness of the little and ring fingers and atrophy of the hypothenar muscles.

**✓ Correct Answer: C. Ulnar nerve before division into superficial and deep branches**