Triple deformity of knee is classically seen in -a) Fracture patellab) Tuberculosisc) Rheumatic arthritisd) Rheumatoid arthritis

**Core Concept**
The triple deformity of the knee refers to a combination of flexion, posterior subluxation, and external rotation of the tibia relative to the femur. This complex deformity is typically associated with a specific condition that affects the knee joint.

**Why the Correct Answer is Right**
The triple deformity of the knee is classically seen in tuberculous arthritis, also known as tuberculous osteoarthritis. In this condition, the tuberculosis infection leads to destruction of the articular cartilage and bone, resulting in a flexion deformity due to contraction of the quadriceps and hamstring muscles. The posterior subluxation occurs due to weakening of the posterior cruciate ligament, and the external rotation is a result of the imbalance of the rotational forces around the knee joint. The pathogenesis involves the destruction of the articular cartilage and bone by the Mycobacterium tuberculosis, which is facilitated by the presence of synovial fluid and the lack of oxygen in the deep layers of the joint.

**Why Each Wrong Option is Incorrect**
**Option A:** Fracture patella is not typically associated with the triple deformity of the knee. While a fracture of the patella can lead to deformity and dysfunction, it does not usually result in the complex combination of flexion, posterior subluxation, and external rotation seen in tuberculous arthritis.

**Option B:** Rheumatic arthritis can cause joint deformity and dysfunction, but it is not typically associated with the triple deformity of the knee. Rheumatic arthritis tends to affect multiple joints and is characterized by inflammation and synovitis, rather than the destruction of articular cartilage and bone seen in tuberculous arthritis.

**Option C:** Rheumatic arthritis is a systemic autoimmune disease that can cause joint deformity and dysfunction, but it is not typically associated with the triple deformity of the knee. Rheumatic arthritis tends to affect multiple joints and is characterized by inflammation and synovitis, rather than the destruction of articular cartilage and bone seen in tuberculous arthritis.

**Option D:** Rheumatoid arthritis is a systemic autoimmune disease that can cause joint deformity and dysfunction, but it is not typically associated with the triple deformity of the knee. Rheumatoid arthritis tends to affect multiple joints and is characterized by inflammation and synovitis, rather than the destruction of articular cartilage and bone seen in tuberculous arthritis.

**Clinical Pearl / High-Yield Fact**
Tuberculous arthritis is a classic example of a condition that can cause a complex joint deformity due to the destruction of articular cartilage and bone. The triple deformity of the knee is a characteristic feature of this condition and should be considered in the differential diagnosis of patients with a complex knee deformity.

**Correct Answer: B. Tuberculosis**