**Question:** Boundaries of quadrilateral space include all EXCEPT

A. pectoralis major muscle
B. subscapularis muscle
C. coracobrachialis muscle
D. axillary nerve

**Correct Answer:** D. axillary nerve

**Core Concept:**

The quadrilateral space is a potential space located between the long head of triceps, brachial plexus, axillary artery, and vein. It is a crucial anatomical landmark in upper limb procedures, as it helps prevent injury to the axillary nerve during surgical maneuvers.

**Why the Correct Answer is Right:**

The axillary nerve (C5, C6, C7) is a crucial structure, innervating the deltoid and teres major muscles, as well as providing sensory innervation to the anterior aspect of the arm and hand. In contrast, the other options are part of the boundaries of the quadrilateral space:

A. Pectoralis major muscle: Not included in the boundaries as it is a separate muscle located anterior to the pectoralis minor muscle.
B. Subscapularis muscle: A part of the rotator cuff muscles, the subscapularis muscle is not part of the boundaries of the quadrilateral space.
C. Coracobrachialis muscle: Another muscle of the rotator cuff, coracobrachialis is not part of the boundaries of the quadrilateral space.

**Why Each Wrong Option is Incorrect:**

A. Pectoralis major muscle: Not included in the boundaries as mentioned above.
B. Subscapularis muscle: As explained, this muscle is not part of the boundaries of the quadrilateral space.
C. Coracobrachialis muscle: Similar to subscapularis, coracobrachialis is not part of the boundaries of the quadrilateral space.

**Clinical Pearl:**

Understanding the boundaries of the quadrilateral space is crucial when performing procedures in the upper limb, as it helps prevent iatrogenic injury to the axillary nerve and its associated structures. Always remember to identify and protect these structures during procedures in the upper limb to minimize complications and ensure patient safety.

**Why the Correct Answer is Right:**

In the context of the quadrilateral space, the axillary nerve is an essential consideration due to its role in innervating the deltoid and teres major muscles, as well as providing sensory innervation to the anterior arm and hand. The correct answer (axillary nerve) is not part of the boundaries of the quadrilateral space, unlike the other options which are muscles of the rotator cuff.