## **Core Concept**
The question assesses knowledge of pediatric orthopedics, specifically the common causes of limp in children. A limp in a child can be caused by various factors including infections, trauma, and congenital conditions. The differential diagnosis varies based on age.

## **Why the Correct Answer is Right**
The most common cause of limp in a child of seven years old is **Perthes disease (C)**, also known as Legg-Calve-Perthes disease. This condition is characterized by avascular necrosis of the femoral head, leading to fragmentation and potential deformation of the femoral head. It typically presents in children between the ages of 4 and 10 years, with a peak incidence around 6-7 years. The exact cause is unknown, but it is believed to result from transient interruption of blood supply to the femoral head.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Infection (e.g., septic arthritis)** is a critical and urgent cause of acute limp in children, it is not the most common cause in a 7-year-old without specifying acute presentation. Infections can occur at any age but are more commonly seen in younger children.
- **Option B:** **Trauma** is indeed a cause of limp but usually presents acutely and might not be the most common reason for a chronic limp in a 7-year-old without a history of recent trauma.
- **Option D:** **Slipped Capital Femoral Epiphysis (SCFE)** is another important cause of limp in children, particularly in the pre-teen years, more commonly around 10-14 years old, making it less likely in a 7-year-old compared to Perthes.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when evaluating a child with a limp, it's crucial to consider the age of the child to narrow down the differential diagnoses. For a 7-year-old child, Perthes disease should be high on the list. Always assess for pain, limitation of motion, and potential for systemic symptoms like fever, which might point towards infection.

## **Correct Answer:** C. Perthes disease.