## 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. The main components of the rotator cuff are the **supraspinatus**, **infraspinatus**, **teres minor**, and **subscapularis** muscles.

## Why the Correct Answer is Right
The muscles that make up the rotator cuff are specifically those that stabilize the glenohumeral joint. The **supraspinatus**, **infraspinatus**, **teres minor**, and **subscapularis** muscles are well-known for their role in shoulder stability and mobility. The **pectoralis major** muscle, however, is not considered part of the rotator cuff. It is a large muscle of the chest that plays a role in movements like flexion, extension, and adduction of the shoulder joint but is not directly involved in the stabilization of the glenohumeral joint in the same way as the rotator cuff muscles.

## Why Each Wrong Option is Incorrect
* **Option A:** The **supraspinatus** muscle is one of the rotator cuff muscles, responsible for shoulder abduction and stabilization.
* **Option B:** The **subscapularis** muscle is also a rotator cuff muscle, primarily involved in medial rotation and stabilization of the shoulder.
* **Option C:** The **infraspinatus** and **teres minor** muscles are part of the rotator cuff, contributing to lateral rotation and stabilization.

## Clinical Pearl / High-Yield Fact
A useful clinical correlation to remember is that rotator cuff injuries often present with weakness in specific movements: **supraspinatus** injury affects abduction, **infraspinatus** and **teres minor** injuries affect external rotation, and **subscapularis** injury affects internal rotation. The **pectoralis major** muscle, not being part of the rotator cuff, is often tested separately for its functions in movements like adduction and medial rotation.

## Correct Answer: D. Pectoralis major.