**Core Concept**
Genu valgum, also known as knock-knee deformity, is a condition characterized by an abnormal angulation of the knee joint, resulting in a varus alignment of the lower limbs. This deformity is often associated with various underlying causes, including genetic factors, trauma, and metabolic bone disorders.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of genu valgum, where an imbalance between the medial and lateral condyles of the femur, along with the corresponding tibial condyles, leads to the characteristic valgus deformity. This imbalance can be due to various factors, including unequal growth rates of the medial and lateral femoral condyles, or as a result of conditions like rickets or Blount's disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Blount's disease is a recognized cause of genu varum (bowleggedness), not genu valgum (knock-knee deformity).
* **Option B:** This option is incorrect because rickets is associated with genu varum (bowleggedness) in children, particularly in those with vitamin D deficiency.
* **Option C:** This option is incorrect because conditions like poliomyelitis can lead to genu varum (bowleggedness) due to muscle imbalances and weakness.

**Clinical Pearl / High-Yield Fact**
Genu valgum can be a sign of underlying metabolic bone disorders, such as rickets or osteomalacia, particularly in children. It is essential to investigate and address these conditions promptly to prevent long-term complications and deformities.

**Correct Answer:** A. Blount's disease.