**Core Concept**
The radial nerve is a branch of the brachial plexus that runs down the arm, passing through the spiral groove of the humerus. Injury to this nerve can result in weakness or paralysis of the muscles of the posterior arm, forearm, and hand.

**Why the Correct Answer is Right**
Injury to the radial nerve in the lower part of the spiral groove typically affects the nerve after it has given off its branches to the triceps muscle. The radial nerve then continues as the posterior interosseous nerve (PIN), which supplies the muscles of the posterior forearm and the extensor muscles of the wrist and fingers. This type of injury can result in wrist drop, a condition where the wrist cannot be extended due to weakness of the extensor carpi radialis longus and brevis muscles.

**Why Each Wrong Option is Incorrect**

* **Option A:** Injury to the radial nerve in the lower part of the spiral groove typically affects the posterior interosseous nerve (PIN), not the main radial nerve trunk. This is why wrist drop occurs.
* **Option B:** The radial nerve is not primarily affected by injury in the axilla or at the level of the mid-humerus. These locations would more likely affect the axillary nerve or the musculocutaneous nerve, respectively.
* **Option C:** Injury to the radial nerve in the lower part of the spiral groove does not result in a "Saturday night palsy," which is typically caused by compression of the radial nerve at the elbow due to prolonged pressure on the nerve, often occurring when a person falls asleep with their arm hanging over the back of a chair.

**Clinical Pearl / High-Yield Fact**
Radial nerve injuries in the spiral groove can be remembered using the mnemonic "Saturday Night Palsy" is incorrect for this specific injury location, but a more accurate association is with prolonged compression of the nerve, such as from lying on a bent elbow.

**Correct Answer:** C