Okay, let's tackle this question. The patient is a 55-year-old woman with severe shoulder pain during tennis, leading to an inability to initiate abduction but able to continue once the arm is elevated to 45 degrees. The key here is understanding the muscles involved in shoulder abduction.

First, shoulder abduction is primarily done by the deltoid and the supraspinatus. The deltoid has three parts: anterior, middle, and posterior. The middle deltoid is the main abductor. However, the supraspinatus is responsible for initiating the first 15 degrees of abduction before the deltoid takes over. If there's an injury to the supraspinatus, the patient wouldn't be able to start abduction. But once the arm is lifted past that initial 15 degrees, the deltoid can take over.

The scenario here says she can't initiate abduction but can continue once the arm is at 45 degrees. That suggests the supraspinatus is damaged because that's the muscle responsible for the initial movement. The deltoid is intact since she can finish the abduction once started.

Looking at the options, the correct answer would be the supraspinatus. The other muscles like the infraspinatus and teres minor are involved in rotation, not abduction. The subscapularis is also a rotator but not for abduction. The trapezius and serratus anterior are more involved in scapular movement, so they wouldn't directly affect arm abduction.

So the answer is the supraspinatus. The clinical pearl here is remembering the "empty can" test for supraspinatus injury and the role of each muscle in the initial phase of abduction.

**Core Concept**
Shoulder abduction is a two-phase process: the *supraspinatus* initiates the first 15° of motion, while the *deltoid* completes the remaining 90°. This question tests knowledge of rotator cuff muscle functions and their roles in shoulder mechanics.

**Why the Correct Answer is Right**
The **supraspinatus** is the primary muscle responsible for initiating arm abduction from the 0° to 15° position. When it is injured (e.g., due to rotator cuff tear), the patient cannot start abduction independently. However, once the arm is passively elevated to ~45°, the deltoid can take over to complete the movement. This "abduction paradox" is classic for supraspinatus pathology, as seen in this patient’s inability to initiate but ability to continue after 45°.

**Why Each Wrong Option is Incorrect**
**Option A: Deltoid** – Incorrect. The deltoid completes abduction after the first 15°, so its injury would prevent *full* abduction, not just initiation.
**Option B: Infraspinatus** – Incorrect. This muscle is a primary external rotator, not involved in abduction.
**Option C: Teres minor** – Incorrect. Like the infraspinatus, it aids external rotation, not abduction.
**Option D: Subscapularis** – Incorrect. This muscle is the main internal rotator; its injury would not affect abduction mechanics.

**Clinical Pearl**
Remember the **"empty can" test** (Jobe’s test) for supraspinatus injury: resisted abduction

✓ Correct Answer: C. Supraspinatus