Okay, let's tackle this question about the teres minor muscle. The question is asking which nerve supplies it. First, I need to recall the anatomy of the shoulder muscles. The teres minor is one of the rotator cuff muscles, right? The rotator cuff includes the supraspinatus, infraspinatus, teres minor, and subscapularis. Each of these is innervated by different nerves.

The teres minor specifically is involved in the external rotation of the shoulder. I remember that the axillary nerve supplies the deltoid and teres minor. Wait, the axillary nerve comes from the posterior cord of the brachial plexus. Let me double-check. Yes, the axillary nerve (C5, C6) innervates both the deltoid and the teres minor. The other rotator cuff muscles have different innervations: supraspinatus and infraspinatus are by the suprascapular nerve, and subscapularis by the subscapular nerves.

Now, looking at the options (though they're not listed here), the common distractors would be other nerves like the musculocutaneous, radial, or ulnar. The musculocutaneous nerve supplies the biceps brachii, so that's not it. The radial nerve innervates the triceps and extensors of the forearm. The ulnar nerve is for the intrinsic hand muscles and some forearm flexors. So the correct answer here is the axillary nerve.

Wait, another thing: the axillary nerve also gives a cutaneous branch to the skin over the deltoid. Damage to this nerve can lead to both weakness in shoulder abduction and sensory loss in that area. So if the question had options including axillary nerve, that's the right choice. The other options are incorrect because they supply different muscles. The clinical pearl here is that the teres minor and deltoid share the same nerve supply, and damage to the axillary nerve would affect both. That's a key point for exams.

**Core Concept** The teres minor is a rotator cuff muscle responsible for shoulder external rotation, and its innervation is a key clinical point in diagnosing brachial plexus injuries. The axillary nerve (C5-C6) supplies it, originating from the posterior cord of the brachial plexus.

**Why the Correct Answer is Right** The axillary nerve innervates the teres minor and deltoid muscles. It arises from the posterior cord (C5-C6) and is vulnerable to injury from shoulder dislocations or humeral fractures. Damage to this nerve causes weakness in shoulder abduction (deltoid) and external rotation (teres minor), with sensory loss over the lateral shoulder.

**Why Each Wrong Option is Incorrect**
**Option A:** *Musculocutaneous nerve* supplies biceps brachii and brachialis—not shoulder rotator cuff muscles.
**Option B:** *Radial nerve* innervates triceps and extensor muscles of the forearm, not the teres minor.
**Option C:** *Ulnar nerve* supplies intrinsic hand muscles and some forearm flexors, unrelated to shoulder external rotators.

**Clinical Pearl / High-Yield Fact** Remember the "A for Axillary" rule: axillary nerve innervates **A**

✓ Correct Answer: D. Axillary nerve