**Core Concept**
Genu valgum, also known as knock knees, is a common condition in children characterized by an abnormal angulation of the knee joint, resulting in a noticeable gap between the knees when the child stands with their feet together. This condition is often caused by an imbalance in the growth and development of the bones and cartilage around the knee joint.

**Why the Correct Answer is Right**
The most common cause of genu valgum in children is excessive growth of the medial (inner) aspect of the distal femur or proximal tibia, leading to a relative shortening of the lateral (outer) aspect of the distal femur or proximal tibia. This results in an inward angulation of the knee joint, causing the knees to touch or overlap when the child stands with their feet together. The growth plates of the bones around the knee joint, particularly the distal femur and proximal tibia, are responsible for this excessive growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteochondritis dissecans is a condition that affects the cartilage and bone in joints, but it is not a common cause of genu valgum in children.
**Option B:** Congenital conditions such as Blount's disease can cause genu varum (bowlegs), not genu valgum (knock knees).
**Option C:** Rheumatoid arthritis is a chronic autoimmune disorder that can cause joint deformities, but it is not a common cause of genu valgum in children.

**Clinical Pearl / High-Yield Fact**
Genu valgum in children is often a self-limiting condition that resolves as the child grows and develops, with the knees eventually returning to a more normal alignment.

**Correct Answer:** C.