**Core Concept**
The **rotator cuff** is a group of muscles and tendons that surround the **shoulder joint**, providing stability and facilitating movement. The **subscapularis muscle** is one of the four muscles of the rotator cuff, along with the **supraspinatus**, **infraspinatus**, and **teres minor**.

**Why the Correct Answer is Right**
The **subscapularis muscle** is often referred to as the "forgotten muscle" of the rotator cuff because it is frequently overlooked in clinical evaluations and rehabilitation protocols. This muscle plays a crucial role in **internal rotation** and **stabilization** of the shoulder joint.

**Why Each Wrong Option is Incorrect**
**Option A:** Supraspinatus muscle is not typically referred to as the forgotten muscle, as it is commonly implicated in **impingement syndrome** and **rotator cuff tears**.
**Option B:** Infraspinatus muscle is also not the forgotten muscle, as it is involved in **external rotation** and is often assessed in clinical examinations.
**Option C:** Teres minor muscle, while sometimes neglected, is not the primary muscle referred to as the forgotten muscle.

**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** is not provided, however, the subscapularis is the correct answer.

**Clinical Pearl / High-Yield Fact**
The **subscapularis muscle** is a key component of the rotator cuff, and its dysfunction can lead to significant **shoulder instability** and **decreased range of motion**.

**Correct Answer:** D. Subscapularis