**Core Concept**
Cubitus valgus deformity is a type of malunion of the elbow joint, characterized by a varus angulation of the distal fragment and a valgus angulation of the proximal fragment. This deformity results from improper healing of the fracture or disruption of the medial collateral ligament.

**Why the Correct Answer is Right**
The most common cause of cubitus valgus deformity is a supracondylar fracture of the humerus, particularly type III fractures with a high risk of nonunion or malunion. During the healing process, the distal fragment tends to drift away from the midline, resulting in a valgus deformity. This is often due to the disruption of the medial collateral ligament and the lateral condyle's inability to support the elbow joint properly.

**Why Each Wrong Option is Incorrect**
**Option A:** Is not directly related to cubitus valgus deformity, although it may be associated with other complications such as stiffness or nerve damage.
**Option B:** Is not a common cause of cubitus valgus deformity, as it typically results in a different type of deformity.
**Option C:** Is not directly associated with cubitus valgus deformity, although it may be related to other elbow injuries.

**Clinical Pearl / High-Yield Fact**
Cubitus valgus deformity can be prevented by ensuring proper reduction and immobilization of supracondylar fractures, particularly in the pediatric population where growth plates are still developing.

**Correct Answer: C. Supracondylar fracture of the humerus**