**Core Concept**
Tardy ulnar nerve palsy refers to a delayed onset of ulnar nerve dysfunction, typically occurring years after a traumatic or repetitive injury to the elbow. This condition is characterized by a gradual onset of ulnar nerve symptoms, often in the presence of a pre-existing cubitus valgus deformity (carrying angle).

**Why the Correct Answer is Right**
The development of tardy ulnar nerve palsy is attributed to the progressive stretching and compression of the ulnar nerve, which occurs as a result of the cubitus valgus deformity. The increased tension on the ulnar nerve leads to a gradual loss of nerve function, resulting in symptoms such as weakness, numbness, and atrophy of the ulnar-innervated muscles. The ulnar nerve is particularly susceptible to damage in the presence of a cubitus valgus deformity due to its anatomical course, which makes it more prone to stretching and compression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a clear cause for tardy ulnar nerve palsy. While trauma can cause ulnar nerve damage, it is not the primary cause of tardy ulnar nerve palsy.
**Option B:** This option is incorrect as it implies that tardy ulnar nerve palsy is a direct result of a fracture, which is not the case. Tardy ulnar nerve palsy can occur years after a fracture, and is often associated with a pre-existing cubitus valgus deformity.
**Option C:** This option is incorrect as it suggests that tardy ulnar nerve palsy is a congenital condition. While some cases of ulnar nerve damage may be present at birth, tardy ulnar nerve palsy is typically a acquired condition that develops over time.

**Clinical Pearl / High-Yield Fact**
A key feature of tardy ulnar nerve palsy is the presence of a pre-existing cubitus valgus deformity, which can be a sign of a previous fracture or injury to the elbow. This condition highlights the importance of careful evaluation and monitoring of patients with a history of elbow trauma.

**Correct Answer: D. A previous fracture of the medial epicondyle or a cubitus valgus deformity.**