**Core Concept**
Septic arthritis in a child is a medical emergency that requires prompt diagnosis and treatment. It occurs when a bacterial or fungal infection enters the joint space, leading to inflammation, pain, and loss of joint function. The most common causative organisms are Staphylococcus aureus and Haemophilus influenzae.

**Why the Correct Answer is Right**
In a 3-year-old child presenting with a limp, septic arthritis is a primary concern. The child's age and the acute onset of symptoms suggest a bacterial infection. The knee joint is the most commonly affected joint in children, and septic arthritis can cause severe pain, swelling, and decreased range of motion. The diagnosis is often made based on clinical presentation, laboratory tests (e.g., elevated white blood cell count, C-reactive protein), and imaging studies (e.g., X-rays, ultrasound). Prompt treatment with antibiotics and joint aspiration is essential to prevent long-term joint damage.

**Why Each Wrong Option is Incorrect**

**Option A:** Slipped capital femoral epiphysis (SCFE) is a hip condition that typically occurs in older children and adolescents, often after a period of rapid growth. While it can cause a limp, it is less common in 3-year-olds.

**Option B:** Perthes disease is a condition that affects the hip joint in children, but it typically presents with hip pain and limited mobility, rather than a limp. It is also more common in older children.

**Option C:** Developmental dysplasia of the hip (DDH) is a congenital condition that affects the hip joint, but it typically presents at birth or in infancy, rather than in early childhood.

**Clinical Pearl / High-Yield Fact**
In pediatric patients presenting with a limp, it is essential to consider septic arthritis as a possible diagnosis, especially if the child is under 5 years old and has a fever, pain, or swelling in the affected joint. Prompt diagnosis and treatment can prevent long-term joint damage and disability.

**✓ Correct Answer: A. Septic Arthritis**