**Question:** All of the following are responsible for internal rotation of shoulder joint EXCEPT

A. Subscapularis muscle
B. Infraspinatus muscle
C. Teres minor muscle
D. Deltoid muscle

**Core Concept:**
The shoulder joint is a ball-and-socket joint, consisting of the humerus (upper arm bone) and the scapula (shoulder blade). The rotator cuff muscles, which include the subscapularis, infraspinatus, teres minor, and deltoid muscles, play a crucial role in stabilizing and moving the shoulder joint. Internal rotation of the shoulder joint refers to rotating the forearm towards the body, which is primarily performed by the rotator cuff muscles.

**Why the Correct Answer is Right:**
The deltoid muscle is the largest and strongest of the rotator cuff muscles, primarily responsible for abduction, adduction, and flexion of the shoulder joint. Deltoid muscle does not play a significant role in internal rotation.

**Why Each Wrong Option is Incorrect:**
A. Subscapularis muscle: This muscle is primarily responsible for internal rotation of the shoulder joint. It is a crucial muscle for internal rotation in the intact shoulder joint.
B. Infraspinatus muscle: This muscle is involved in external rotation of the shoulder joint but also plays a minor role in internal rotation.
C. Teres minor muscle: This muscle is involved in both external and internal rotation of the shoulder joint, but its role in internal rotation is less significant compared to subscapularis.

**Why D is Incorrect:**
D. Deltoid muscle: As mentioned earlier, deltoid muscle is primarily involved in abduction, adduction, and flexion of the shoulder joint but plays minimal role in internal rotation.

**Clinical Pearl:**
Understanding the specific roles of rotator cuff muscles in joint stability and movement is essential for clinical practice, particularly in diagnosing and treating shoulder joint disorders. A comprehensive knowledge of these muscles and their functions is critical for accurate diagnosis and appropriate treatment strategies.