**Core Concept**
Tardy ulnar nerve palsy is a delayed complication of nerve compression or injury, most commonly occurring in children due to fracture of the lateral condyle of the humerus. This fracture disrupts the normal anatomy of the elbow joint and can lead to mechanical impingement of the ulnar nerve as it passes through the cubital tunnel.

**Why the Correct Answer is Right**
In children, a fracture of the lateral condyle of the humerus (commonly known as a "greenstick" or "extension" fracture) can cause displacement of the bone and disruption of the normal alignment of the elbow. This leads to compression of the ulnar nerve as it passes through the cubital tunnel. The palsy develops gradually over weeks to months due to chronic mechanical irritation, not immediate trauma. This is known as "tardy" ulnar nerve palsy and is a classic sequela of such fractures in pediatric patients.

**Why Each Wrong Option is Incorrect**
Option A: Supracondylar fracture of humerus typically causes radial nerve injury or elbow flexion contractures, not ulnar nerve palsy. It does not compress the ulnar nerve in a way that leads to delayed palsy.
Option B: Posterior dislocation of elbow can cause radial nerve injury (from compression at the radial groove), but ulnar nerve palsy is rare and not typically delayed.
Option D: Olecranon fracture may cause elbow instability and nerve injury, but the ulnar nerve is not commonly compressed in this setting, and palsy is usually acute rather than delayed.

**Clinical Pearl / High-Yield Fact**
In children, lateral condyle fractures are a common cause of delayed ulnar nerve palsy due to mechanical impingement. Early diagnosis and proper reduction are essential to prevent long-term motor and sensory deficits. Always assess for ulnar nerve signs (e.g., claw hand, loss of hand flexion) in children with elbow trauma.

✓ Correct Answer: C. Fracture of lateral condyle of humerus in children