**Core Concept:** The glenohumeral (GH) joint is a ball-and-socket joint, which allows a wide range of motion. Internal rotation is one of the movements at the GHJ, and the muscles responsible for this movement are specific rotator muscles.

**Why the Correct Answer is Right:** The rotator muscles that perform internal rotation at the GHJ are the subscapularis, teres minor, and pectoralis minor muscles. These muscles act together to rotate the humerus internally. The subscapularis and teres minor muscles are the primary rotator muscles, while the pectoralis minor assists by stabilizing the scapula.

**Why Each Wrong Option is Incorrect:**

A. **Infraspinatus:** This muscle is part of the rotator cuff, which primarily stabilizes the humerus at the GHJ, rather than rotating it internally.

B. **Supraspinatus:** Similar to infraspinatus, supraspinatus primarily stabilizes the humerus, not internally rotate it.

C. **Deltoid:** The deltoid muscle is responsible for shoulder elevation, abduction, and anterior rotation at the GHJ, not internal rotation.

D. **Pectoralis Minor:** Although this muscle is involved in stabilizing the scapula, its primary function is to elevate the clavicle, not perform internal rotation at the GHJ.

**Clinical Pearl:** Understanding the specific muscles responsible for internal rotation at the GHJ is crucial for accurate physical examination findings and differential diagnosis in clinical practice. A positive internal impingement test can suggest an internal impingement syndrome, which may indicate rotator cuff tears or other pathology.

**Correct Answer:** D. Pectoralis Minor.

Pectoralis Minor is the correct answer because it plays a role in stabilizing the scapula, which indirectly aids in internal rotation of the humerus at the glenohumeral joint. The other options are incorrect because they primarily function in stabilization or other shoulder movements rather than internal rotation.