**Question:** All of the following forms the boundaries of the lower triangular space of arm, EXCEPT:

A. Subscapularis muscle
B. Anterolateral arm
C. Supraspinatus muscle
D. Anteromedial arm

**Core Concept:**
The lower triangular space of the arm is a potential space located inferior to the subscapular fossa and medial to the deltoid muscle. It is bounded by the subscapularis muscle, the coracoid process, and the humerus. The arm's anatomy involves various muscles, bones, and ligaments, which play a crucial role in defining the space's boundaries.

**Why the Correct Answer is Right:**
The correct answer, D) Anteromedial arm, is not part of the lower triangular space of the arm because it refers to the anteromedial aspect of the arm, which is outside the boundaries of the triangular space. The space lies medial to the deltoid muscle and is bounded by the subscapularis muscle and the coracoid process of the scapula.

**Why Each Wrong Option is Incorrect:**

A) Subscapularis muscle: This option is incorrect because it belongs to the lower triangular space, defining one of its boundaries. The subscapularis muscle is responsible for flexing and internal rotation of the shoulder joint and is a key component of the lower triangular space.

B) Anterolateral arm: This option is incorrect because it describes the anterolateral aspect of the arm, which is not part of the lower triangular space. The space lies inferior to the subscapular fossa and medial to the deltoid muscle.

C) Supraspinatus muscle: This option is incorrect because the supraspinatus muscle is part of the rotator cuff muscles that stabilize the humeral head of the glenohumeral joint, and it is not a boundary of the lower triangular space of the arm.

**Clinical Pearl:**
The understanding of the lower triangular space of the arm is essential for identifying and diagnosing conditions within the space, such as infections, abscesses, or tumors. A thorough knowledge of the arm's anatomy helps doctors to accurately assess and treat patients' conditions involving the lower triangular space or surrounding structures.