**Core Concept**
A Colles' fracture is an osteoporotic fracture of the distal radius, typically occurring in elderly women after a fall onto an outstretched hand. The fracture leads to disruption of the normal anatomical alignment and function of the wrist.

**Why the Correct Answer is Right**
The commonest complication of a Colles' fracture is **malunion**. Malunion occurs when the fracture heals in an abnormal position, leading to a persistent deformity. This is due to the displacement of the fracture fragments, which can be caused by the force of the fall or the natural tendency of the bone to return to its original position. The displacement of the fracture fragments can lead to a persistent shortening, angulation, or rotation of the distal fragment, resulting in a malunited Colles' fracture.

**Why Each Wrong Option is Incorrect**
**Option A:** **Nerve injury** is a possible complication of Colles' fracture, but it is not the commonest complication. Nerve injury can occur due to the displacement of the fracture fragments or the swelling and inflammation that follows the injury. However, nerve injury is relatively rare compared to malunion.
**Option B:** **Infection** is a potential complication of any fracture, but it is not the commonest complication of Colles' fracture. Infection can occur due to the introduction of bacteria into the wound or the soft tissues surrounding the fracture. However, the risk of infection is relatively low with Colles' fracture.
**Option C:** **Vascular compromise** is not a common complication of Colles' fracture. Vascular compromise can occur due to the displacement of the fracture fragments or the swelling and inflammation that follows the injury. However, vascular compromise is relatively rare and is not the commonest complication of Colles' fracture.

**Clinical Pearl / High-Yield Fact**
The treatment of Colles' fracture typically involves closed reduction and immobilization in a cast or splint. However, if the fracture is significantly displaced or comminuted, open reduction and internal fixation may be required to achieve proper alignment and healing.

**Correct Answer:** A. Malunion.