**Core Concept**
Colles' fracture is a type of fracture at the distal end of the radius in the forearm, characterized by a displacement of the bone fragment. It is typically caused by a fall onto an outstretched hand, resulting in a dorsal angulation of the distal fragment. The complications of Colles' fracture can be both short-term and long-term, affecting the functional outcome and quality of life of the patient.

**Why the Correct Answer is Right**
Non-union is an unlikely complication of Colles' fracture due to the abundance of blood supply to the distal radius. The distal radius has a rich network of blood vessels that facilitate healing and minimize the risk of non-union. In contrast, malunion, Sudeck's dystrophy, and rupture of the extensor pollicis longus are recognized complications of Colles' fracture. Malunion refers to the improper healing of the fracture, resulting in a poor alignment of the bone fragments. Sudeck's dystrophy, also known as complex regional pain syndrome, is a condition characterized by chronic pain, swelling, and changes in skin temperature and color following a traumatic injury. Rupture of the extensor pollicis longus tendon can occur due to the altered mechanics of the wrist after a Colles' fracture.

**Why Each Wrong Option is Incorrect**
**Option A:** Malunion is a recognized complication of Colles' fracture, where the bone fragments do not heal properly, resulting in a poor alignment of the wrist.
**Option C:** Sudeck's dystrophy is a known complication of Colles' fracture, characterized by chronic pain, swelling, and changes in skin temperature and color.
**Option D:** Rupture of the extensor pollicis longus tendon is a possible complication of Colles' fracture, due to the altered mechanics of the wrist following the injury.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the risk of complications following a Colles' fracture can be minimized with proper management, including early mobilization, splinting, and surgical intervention when necessary.

**✓ Correct Answer: B. Non union**