**Core Concept**
Thoracic outlet syndrome (TOS) is a condition characterized by compression of the nerves and/or blood vessels in the thoracic outlet, which is the space between the collarbone (clavicle) and the first rib. The compression of the nerves in this region can lead to various symptoms, including pain, numbness, and weakness in the arm.

**Why the Correct Answer is Right**
The compression of the nerves in thoracic outlet syndrome, particularly in the presence of a cervical rib, typically affects the lower brachial plexus. The most commonly affected nerves are the C8 and T1 nerve roots, which are responsible for innervating the muscles of the forearm and hand. The compression of these nerves leads to symptoms such as numbness and weakness in the hand, particularly in the ulnar distribution.

**Why Each Wrong Option is Incorrect**
**Option A:** The compression of the nerves in thoracic outlet syndrome is not typically associated with the upper brachial plexus, which includes the C5 and C6 nerve roots.

**Option B:** The compression of the nerves in thoracic outlet syndrome is not typically associated with the lumbar plexus, which includes the L1-L4 nerve roots.

**Option C:** The compression of the nerves in thoracic outlet syndrome is not typically associated with the sacral plexus, which includes the S1-S4 nerve roots.

**Clinical Pearl / High-Yield Fact**
A cervical rib is a common anatomical variation that can predispose individuals to thoracic outlet syndrome. It is essential to identify and manage this condition promptly to prevent long-term nerve damage.

**Correct Answer: C. C8 and T1 nerve roots.**