**Core Concept**
A Colles' fracture is a type of distal radius fracture that occurs at the wrist. After a Colles' fracture, the extensor pollicis longus (EPL) tendon can become entrapped in the fracture site, leading to a loss of thumb extension due to the mechanical obstruction of the tendon's normal path.

**Why the Correct Answer is Right**
In a Colles' fracture, the EPL tendon can become entrapped in the fracture site, particularly between the distal fragments of the radius bone. This entrapment prevents the EPL tendon from contracting and extending the thumb, resulting in a loss of thumb extension. The EPL tendon is responsible for extending the thumb at the interphalangeal joint.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the clinical scenario of a Colles' fracture and loss of thumb extension.

**Option B:** This option may be a consideration in other types of wrist injuries or conditions, but it is not directly related to the entrapment of the EPL tendon in a Colles' fracture.

**Option D:** This option may be a consideration in other types of nerve injuries or conditions, but it is not directly related to the entrapment of the EPL tendon in a Colles' fracture.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the EPL tendon entrapment in patients with a Colles' fracture who present with a loss of thumb extension, as this is a common and treatable cause of this symptom.

**Correct Answer: C. EPL tendon entrapment**