## **Core Concept**
The rotator cuff is a group of muscles and their tendons that surround the shoulder joint, keeping the head of the humerus firmly within the shallow socket of the shoulder. It is composed of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis.

## **Why the Correct Answer is Right**
The muscles that form the rotator cuff are specifically the supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles work together to stabilize the shoulder joint and facilitate a wide range of movements. The correct answer, teres major, does not belong to this group; instead, it is involved in adduction and internal rotation of the humerus.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Supraspinatus - This muscle is one of the four muscles that form the rotator cuff. It helps to stabilize the shoulder joint and assist in abduction of the arm.
* **Option B:** Subscapularis - This is also one of the four muscles of the rotator cuff. It helps in medial rotation and stabilization of the shoulder joint.
* **Option C:** Infraspinatus - Another muscle of the rotator cuff, it assists in lateral rotation and stabilization of the shoulder joint.

## **Clinical Pearl / High-Yield Fact**
A useful clinical correlation to remember is that rotator cuff injuries often present with pain and weakness in movements that involve abduction (lifting the arm out to the side) and external rotation. The teres major, being a separate entity, can be a source of confusion but remembering its role in adduction helps differentiate it from the rotator cuff muscles.

## **Correct Answer:** D. Teres major.