**Question:** All of the following structures forms the boundaries of the quadrangular space which is present under the deltoid muscle in the shoulder region, from the posterior side, EXCEPT?

A. Subscapularis
B. Supraspinatus
C. Suprascapular Nerve
D. Teres minor

**Correct Answer:** **Correct Answer: C. Suprascapular Nerve**

**Core Concept:**
The quadrangular space is a potential space located under the deltoid muscle in the posterior aspect of the shoulder joint. It is formed by the boundaries of the following muscles: supraspinatus, subscapularis, and teres minor. The suprascapular nerve, however, does not form part of the boundaries of the quadrangular space.

**Why the Correct Answer is Right:**
The quadrangular space is a region in the shoulder where the deltoid muscle lies posteriorly and the subscapularis, supraspinatus, and teres minor muscles surround it. The suprascapular nerve is an important structure that arises from the C5 and C6 cervical spinal nerves and is responsible for supplying sensation to the posterior aspect of the shoulder joint and motor innervation to the supraspinatus and teres minor muscles. However, the suprascapular nerve itself is not a part of the boundaries of the quadrangular space.

**Why Each Wrong Option is Incorrect:**
A. Subscapularis: This muscle is one of the three muscles forming the boundaries of the quadrangular space, so it is not the correct answer.
B. Supraspinatus: Similarly to subscapularis, the supraspinatus muscle is part of the boundaries of the quadrangular space, so it is incorrect.
D. Teres minor: This muscle is also involved in forming the boundaries of the quadrangular space, making it the incorrect answer.

**Clinical Pearl:**
Understanding the anatomy and relationships of the quadrangular space is crucial for safe and effective clinical procedures in the shoulder region, such as injections and surgeries. Familiarizing yourself with the structures involved in this space helps prevent iatrogenic injury to the suprascapular nerve during these procedures.