**Core Concept**
Triple deformity of the knee refers to a combination of three abnormal movements of the knee joint: flexion, posterior subluxation, and external rotation. This condition is often associated with severe injuries or conditions that affect the knee's ligamentous and muscular stability.

**Why the Correct Answer is Right**
The triple deformity of the knee typically occurs as a result of a severe knee injury, such as a high-velocity trauma or a severe valgus force. This type of injury can cause the tibia to subluxate posteriorly, leading to a flexion deformity. The external rotation of the tibia is due to the disruption of the lateral collateral ligament and the biceps femoris muscle. The combination of these deformities results in a triple deformity of the knee.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a condition that can cause triple deformity of the knee. While patellar fractures can cause knee instability, they are not typically associated with triple deformity.
* **Option B:** This option is incorrect because it is a condition that affects the hip joint, not the knee joint. Developmental dysplasia of the hip can cause hip instability and deformity, but it is not associated with triple deformity of the knee.
* **Option C:** This option is incorrect because it is a condition that affects the ankle joint, not the knee joint. Ankle fractures can cause ankle instability and deformity, but they are not associated with triple deformity of the knee.

**Clinical Pearl / High-Yield Fact**
In cases of severe knee trauma, it is essential to assess for the presence of a triple deformity, as it can indicate a more severe injury that may require surgical intervention.

**Correct Answer:** D.