**Coxa Vara**

**Core Concept**
Coxa vara is a congenital or acquired deformity of the hip joint characterized by a reduced angle between the femoral neck and shaft, resulting in a shortening and outward angulation of the lower limb. This condition can lead to gait disturbances, pain, and decreased mobility.

**Why the Correct Answer is Right**
The correct answer is **Perthes disease**. Perthes disease, also known as Legg-Calve-Perthes disease, is a childhood hip disorder that affects the femoral head, leading to aseptic necrosis and subsequent deformation of the hip joint. In some cases, Perthes disease can cause a coxa vara deformity due to the collapse of the femoral head and subsequent shortening of the femoral neck.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Slipped capital femoral epiphysis (SCFE)** is a condition where the femoral head slips off the femoral neck, but it does not typically cause a coxa vara deformity.
* **Option B:** **Developmental dysplasia of the hip (DDH)** is a condition where the hip joint does not form properly, but it is not directly associated with a coxa vara deformity.
* **Option C:** **Osteonecrosis of the femoral head** can cause a variety of deformities, but it is not a primary cause of coxa vara.

**Clinical Pearl / High-Yield Fact**
Perthes disease often presents with hip pain, limited mobility, and a limp in children between the ages of 4 and 10 years. Early diagnosis and treatment are crucial to prevent long-term complications and deformities.

**Correct Answer:** D. Perthes disease.