**Core Concept**
Thalassemia is a genetic disorder characterized by defective hemoglobin production, leading to various systemic complications, including bone and joint abnormalities. The pathophysiology involves bone marrow expansion, increased erythropoiesis, and subsequent bone deformities. In thalassemia, the bone changes are primarily due to the increased bone marrow space occupied by the expanded bone marrow, leading to bone thinning and deformation.

**Why the Correct Answer is Right**
The knee joint is frequently affected in thalassemia due to the characteristic bone marrow expansion and subsequent bone deformities. This is primarily due to the increased bone marrow space in the long bones of the lower limbs, including the femur and tibia. The bone thinning and deformation lead to characteristic changes in the knee joint, including genu varum (bow-legged deformity) and genu valgum (knock-knee deformity). The knee joint is more commonly affected compared to other joints like the hip, shoulder, or ankle.

**Why Each Wrong Option is Incorrect**
**Option A:** The hip joint is less commonly involved in thalassemia due to its relatively smaller bone marrow space compared to the knee joint. While hip involvement can occur, it is less frequent than knee involvement.
**Option C:** The shoulder joint is not typically associated with thalassemia-related bone deformities. The bone changes in thalassemia primarily affect the long bones of the lower limbs.
**Option D:** The ankle joint is not commonly involved in thalassemia. The characteristic bone marrow expansion and bone deformities primarily affect the knee joint.

**Clinical Pearl / High-Yield Fact**
In thalassemia, the characteristic bone deformities are primarily due to bone marrow expansion and subsequent bone thinning. The knee joint is frequently involved, and a high index of suspicion should be maintained for knee deformities in patients with thalassemia.

✓ Correct Answer: B. Knee