**Question:** Commonest complication of Colles fracture

Core Concept: A Colles fracture is a type of distal radius fracture, typically caused by a fall onto an outstretched hand. Common complications include improper healing, malunion, and non-union.

**Why the Correct Answer is Right:** The most common complication of a Colles fracture is malunion, which occurs when the bones do not heal in the correct alignment. This can lead to chronic pain, decreased range of motion, and increased risk of arthritis in the future.

**Why Each Wrong Option is Incorrect:**

A. Non-union: While non-union is also a complication of Colles fractures, it is less common than malunion. Non-union occurs when the fracture fails to heal at all, leading to the persistence of symptoms and possible surgical intervention.

B. Union: Union refers to the successful healing of a fracture, which is not the correct answer as we are discussing complications.

C. Arthritis: While Colles fracture healing can lead to increased risk of arthritis, arthritis itself is not a complication, but rather a long-term consequence of the injury.

D. Flexion deformity: This refers to the fracture healing with the wrist in a flexed position, which is considered a complication and is addressed in the rationale for the correct answer (malunion).

**Correct Answer:** D. Flexion deformity (malunion): Malunion occurs when the bones do not heal in the correct alignment, leading to complications such as chronic pain, decreased range of motion, and increased risk of arthritis in the future. In this case, the correct answer is "flexion deformity" since a malunion results in the wrist being in a flexed position, causing the mentioned complications.