**Core Concept**
Congenital dislocation of the knee, also known as congenital knee dislocation or congenital genu recurvatum, is a rare congenital deformity characterized by a dislocation or subluxation of the knee joint. This condition occurs due to an abnormal development of the knee joint or surrounding soft tissues during fetal development.

**Why the Correct Answer is Right**
The commonest presentation of congenital dislocation of the knee is indeed hyperextension. This is because the ligaments and joint capsule surrounding the knee joint are either absent or poorly developed, leading to an unstable knee joint that can easily dislocate or subluxate. In many cases, the knee joint is held in a hyperextended position due to the abnormal development of the surrounding soft tissues. This hyperextension can be accompanied by a shortening of the lower limb, which can further exacerbate the deformity.

**Why Each Wrong Option is Incorrect**
**Option A:** Varus deformity is not the commonest presentation of congenital dislocation of the knee. While varus deformity can occur in some cases, it is not the typical presentation.
**Option B:** Valgus deformity is also not the commonest presentation of congenital dislocation of the knee. Valgus deformity can occur, but it is not as common as hyperextension.
**Option C:** Flexion deformity is not typically associated with congenital dislocation of the knee. While flexion deformity can occur in some cases, it is not the commonest presentation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that congenital dislocation of the knee is often associated with other congenital anomalies, such as clubfoot or hip dislocation. A thorough examination and imaging studies are necessary to diagnose and manage this condition.

**✓ Correct Answer: D. Hyperextens ion**