**Core Concept**
The triple deformity of the knee, also known as the "genu varum deformity," is a complex condition characterized by three primary deformities: flexion, posterior subluxation, and external rotation of the tibia. This deformity is a result of the abnormal alignment of the knee joint and surrounding soft tissues.

**Why the Correct Answer is Right**
The correct answer, **Flexion, posterior subluxation & external rotation**, accurately describes the triple deformity of the knee. This deformity occurs due to the chronic posterior displacement of the tibia, leading to a flexed position of the knee. The posterior subluxation is a result of the disruption of the posterior cruciate ligament (PCL) and the surrounding soft tissues. The external rotation of the tibia is a consequence of the abnormal alignment of the knee joint and the surrounding muscles.

The mechanism behind this deformity is closely related to the anatomy of the knee joint and the function of the surrounding soft tissues. The PCL plays a crucial role in maintaining the normal alignment of the knee joint, and its disruption leads to the posterior subluxation of the tibia. The external rotation of the tibia is a result of the imbalance of the surrounding muscles, which are no longer able to maintain the normal alignment of the knee joint.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it incorrectly states that the triple deformity of the knee involves extension rather than flexion. Additionally, anterior subluxation, rather than posterior subluxation, is the incorrect description of the deformity.

**Option C:** This option is incorrect because it incorrectly states that the triple deformity of the knee involves internal rotation rather than external rotation. Internal rotation is not a characteristic of the triple deformity of the knee.

**Option D:** This option is incorrect because it incorrectly states that the triple deformity of the knee involves anterior subluxation rather than posterior subluxation. Additionally, internal rotation, rather than external rotation, is the incorrect description of the deformity.

**Clinical Pearl / High-Yield Fact**
A key aspect of the triple deformity of the knee is the disruption of the PCL, which leads to the posterior subluxation of the tibia. This is a critical consideration in the surgical management of this condition, as the PCL must be reconstructed or reinforced to restore normal knee alignment.

**✓ Correct Answer: B. Flexion, posterior subluxation & external rotation**