**Core Concept:** The human body is divided into various spinal cord segments called dermatomes, myotomes, and motor points. Sensory nerves and motor nerves are distributed into these regions. When a nerve is injured, the level of injury can be determined based on the affected area's sensory or motor function.

**Why the Correct Answer is Right:** In this scenario, the patient is experiencing a loss of wrist extension movement but is not experiencing any sensory loss. This suggests that the injury is to a motor nerve, as sensory nerves would be responsible for detecting stimuli rather than producing movement.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because sensory nerves are not responsible for wrist extension movements. As mentioned, sensory nerves are involved in detecting stimuli, not producing movement.

B. This option is incorrect because, as explained above, sensory nerves are not responsible for wrist extension movements.

C. This option is incorrect for the same reason as A and B: sensory nerves are not involved in producing wrist extension movements.

D. This option is correct because motor nerves are responsible for producing movement, and since the patient is experiencing a loss of wrist extension movement, the injury is likely to a motor nerve.

**Clinical Pearl:** In clinical practice, understanding the relationship between dermatomes, myotomes, and motor points is crucial for determining the level of a spinal cord injury based on the patient's symptoms. By correlating the patient's symptoms to the appropriate dermatome or myotome, healthcare providers can make an educated guess about the spinal cord level of injury.

**Correct Answer:** D. Motor point