## **Core Concept**
The quadrangular space, also known as the quadrilateral space, is an anatomical region in the shoulder area that allows for the passage of the axillary nerve and the posterior circumflex humeral artery. It is bounded by specific muscles and bones of the shoulder.

## **Why the Correct Answer is Right**
The quadrangular space is specifically bounded by the following structures:
- Superiorly by the **subscapularis** muscle and **teres minor** muscle
- Inferiorly by the **teres major** muscle
- Medially by the **long head of triceps brachii**
- Laterally by the **surgical neck of the humerus**

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, if a muscle like the subscapularis or teres minor is listed, it could be a correct boundary.
- **Option B:** Similarly, this option cannot be evaluated without content but could represent a correct boundary like the teres major or long head of triceps.
- **Option C:** Without content, we assume it might represent another correct boundary such as the surgical neck of the humerus.
- **Option D:** This is the correct answer because, typically, the boundaries do not include the **pectoralis major** muscle; it is more superficial and not considered a boundary of the quadrangular space.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is the **quadrilateral space syndrome**, which can cause compression of the axillary nerve and posterior circumflex humeral artery, leading to shoulder pain and weakness, particularly affecting the deltoid muscle and teres minor.

## **Correct Answer:** D. pectoralis major.