**Core Concept**
Cubitus varus deformity is a congenital or acquired deformity of the elbow, characterized by a shortened, varus (inwardly deviated) forearm and a decreased carrying angle. This deformity often results from a lateral condylar fracture of the humerus, particularly in children.

**Why the Correct Answer is Right**
The mechanism behind cubitus varus deformity involves the growth arrest of the lateral condyle, leading to an abnormal growth pattern of the humerus. In children, the lateral condyle is responsible for the growth of the forearm. A lateral condylar fracture can disrupt this growth, causing the forearm to become shortened and the elbow to become varus. The deformity is often more pronounced in children due to the ongoing growth and remodeling of the bone.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as cubitus varus deformity is not typically associated with medial epicondylar fractures, which would more likely result in a cubitus valgus deformity.
**Option B:** This option is incorrect as a fracture of the olecranon or the medial epicondyle would not directly result in a cubitus varus deformity.
**Option C:** This option is incorrect as a fracture of the radial head or neck would not directly result in a cubitus varus deformity.

**Clinical Pearl / High-Yield Fact**
Cubitus varus deformity is often associated with a decreased carrying angle, which can lead to functional limitations and decreased quality of life. Early recognition and treatment of lateral condylar fractures in children are essential to prevent the development of this deformity.

**Correct Answer: D. Lateral condylar fracture of the humerus.**