**Core Concept**
Lateral condylar fractures in children are a type of elbow fracture that occurs when the lateral condyle of the humerus is broken. This fracture type is unique in children due to the open growth plates and the presence of the physeal plate, which can affect the healing process and lead to potential complications.

**Why the Correct Answer is Right**
The most common complication of lateral condylar fractures in children is **nonunion**. Nonunion occurs when the fracture fails to heal properly, leading to ongoing pain and instability in the elbow joint. This is due to the physeal plate, which can disrupt the healing process by interfering with blood supply and bone growth. The physeal plate also makes it challenging for the fragments to be reduced and secured properly, increasing the risk of nonunion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Malunion is a possible complication of lateral condylar fractures, but it is not the most common one. Malunion occurs when the fracture heals in an abnormal position, leading to deformity and limited mobility.
* **Option B:** Cubitus varus is a possible complication of lateral condylar fractures, but it is not the most common one. Cubitus varus occurs when the elbow joint becomes misaligned, resulting in a "gunstock" deformity.
* **Option D:** Infection is a possible complication of any fracture, but it is not the most common complication of lateral condylar fractures in children.

**Clinical Pearl / High-Yield Fact**
It is essential for orthopedic surgeons to carefully assess the physeal plate and the fracture fragments in children with lateral condylar fractures to minimize the risk of nonunion.

**Correct Answer: C. Nonunion. Nonunion is the most common complication of lateral condylar fractures in children.**