**Answer:** C. Supraspinatus

**Core Concept:** The rotator cuff is a group of four muscles and their tendons that attach the humerus to the scapula, providing stability and mobility to the shoulder joint. Injuries to these muscles are common due to their frequent use and reliance on overhead movements.

**Why the Correct Answer is Right:** The supraspinatus is the most commonly injured muscle in the rotator cuff because it is responsible for upward rotation and external rotation of the shoulder. This makes it vulnerable during overhead activities and sports involving throwing or reaching.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Infraspinatus):** Although the infraspinatus also contributes to external rotation and abduction, it is less commonly injured due to its deeper location and less frequent use compared to the supraspinatus.

B. **Option B (Teres minor):** The teres minor is responsible for inferior rotation of the humerus, which is less common compared to the actions of supraspinatus and infraspinatus.

D. **Option D (Subscapularis):** The subscapularis is primarily involved in internal rotation and adduction, making it less frequently injured in comparison to the supraspinatus, infraspinatus, and teres minor.

**Clinical Pearl:** Rotator cuff muscle injuries are commonly seen in sports involving repetitive overhead motions, such as swimming, weightlifting, and baseball. Early recognition and treatment are crucial to prevent long-term complications like rotator cuff tendinopathy, tear, or subacromial impingement.

**Correct Answer:** C. Supraspinatus

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In summary, the supraspinatus is the most common injured muscle in the rotator cuff due to its primary role in external rotation and abduction during overhead activities. Understanding the unique functions of each rotator cuff muscle helps in recognizing and treating these injuries promptly, preventing long-term complications.