**Core Concept**
The question requires identifying the muscle responsible for both abduction and internal rotation of the arm. This involves understanding the anatomy and function of the muscles involved in shoulder movements. The rotator cuff muscles play a crucial role in shoulder stability and movement.

**Why the Correct Answer is Right**
The correct answer is the subscapularis muscle, which is part of the rotator cuff. It is responsible for internal rotation of the humerus and assists in abduction. The subscapularis muscle arises from the subscapular fossa and inserts onto the lesser tubercle of the humerus. Its unique function allows it to contribute to both internal rotation and abduction movements of the arm.

**Why Each Wrong Option is Incorrect**
**Option A:** Supraspinatus - Incorrect because the supraspinatus muscle primarily assists in abduction of the arm, but it does not contribute to internal rotation.
**Option B:** Teres minor - Incorrect because the teres minor muscle assists in adduction and external rotation of the arm, not abduction and internal rotation.
**Option C:** Infraspinatus - Incorrect because the infraspinatus muscle assists in external rotation of the arm, not abduction and internal rotation.

**Clinical Pearl / High-Yield Fact**
The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) work together to provide stability and facilitate movement of the shoulder joint. Understanding the function of each muscle is essential for diagnosing and treating shoulder injuries and pathologies.

**Correct Answer:** C. Infraspinatus