In a diving accident that severed the spinal cord below the sixth cervical veebra, which muscle would be affected?
**Core Concept**
The spinal cord is divided into segments, each corresponding to a specific level of the vertebral column. The sixth cervical vertebra (C6) is a significant landmark in the cervical spine, as it marks the transition from the upper to the lower cervical segments. The muscles above this level are primarily innervated by the cervical plexus, while those below are innervated by the brachial plexus.
**Why the Correct Answer is Right**
In a diving accident that severs the spinal cord below C6, the muscles below this level would be affected. This is because the lower cervical and upper thoracic segments are primarily responsible for controlling the muscles of the arm and chest wall. The brachial plexus, which arises from the C5-T1 roots, is responsible for innervating these muscles. Since the spinal cord is severed below C6, the brachial plexus would be affected, leading to weakness or paralysis of the affected muscles.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the question specifies a spinal cord injury below C6, which would not affect the muscles primarily innervated by the cervical plexus.
* **Option B:** This option is incorrect because it is not specified which muscle group is affected, and there are many muscles in the upper limb and chest wall that could be affected by a spinal cord injury below C6.
* **Option D:** This option is incorrect because it is not a specific muscle or muscle group that would be affected by a spinal cord injury below C6.
**Clinical Pearl / High-Yield Fact**
In a spinal cord injury, the level of the injury can be estimated based on the motor and sensory deficits. A spinal cord injury below C6 would typically result in weakness or paralysis of the muscles of the arm and chest wall, while a higher level injury would result in more widespread deficits.
**Correct Answer:** C.