**Core Concept**
The question tests the student's knowledge of the muscles attached to the scapula, specifically those attached to its medial border. The medial border of the scapula is a distinct anatomical landmark that serves as an attachment point for several muscles.

**Why the Correct Answer is Right**
The Rhomboidus Major muscle is a deep muscle of the back that originates from the spinous processes of the lower six thoracic vertebrae (T2-T7) and inserts onto the medial border of the scapula. This muscle plays a crucial role in scapular rotation and stabilization, assisting in movements such as adduction and downward rotation of the scapula. The Rhomboidus Major muscle is one of the primary muscles responsible for this movement, making it the correct answer.

**Why Each Wrong Option is Incorrect**
**Option B:** Teres Major is a muscle attached to the posterior aspect of the scapula, not the medial border. It originates from the lateral border of the scapula and inserts onto the humerus, primarily involved in adduction and internal rotation of the shoulder joint.

**Option C:** Deltoid is a muscle of the shoulder region that originates from the clavicle, acromion, and spine of the scapula, and inserts onto the deltoid tuberosity of the humerus. It is primarily involved in flexion, extension, and rotation of the shoulder joint, but not attached to the medial border of the scapula.

**Option D:** Infraspinatus is a muscle of the rotator cuff that originates from the infraspinous fossa of the scapula and inserts onto the greater tuberosity of the humerus. It is primarily involved in external rotation of the shoulder joint, but not attached to the medial border of the scapula.

**Clinical Pearl / High-Yield Fact**
It is essential to remember the distinct attachments and functions of the various muscles of the scapula, as this knowledge is crucial for understanding and diagnosing shoulder and upper limb disorders.

**✓ Correct Answer: A. Rhomboidus Major**