## **Core Concept**
The question describes a clinical scenario suggestive of rotator cuff injury, specifically involving the muscles responsible for shoulder abduction and external rotation. The rotator cuff is a group of muscles and their tendons that surround the shoulder joint, keeping the head of the humerus firmly within the shallow socket of the shoulder.

## **Why the Correct Answer is Right**
The patient's symptoms, such as weakness in abduction and external rotation of the right arm, inability to hold the arm abducted at 90 degrees, and normal passive range of motion, point towards a rotator cuff pathology. The rotator cuff is primarily composed of four muscles: **supraspinatus**, **infraspinatus**, **teres minor**, and **subscapularis**. The supraspinatus muscle is particularly responsible for the initiation of arm abduction (first 15 degrees), and the infraspinatus along with teres minor facilitates external rotation. Given that the correct answer involves these functions, it likely includes these muscles.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the muscles of the rotator cuff.
- **Option B:** Similarly, this option does not correctly list the muscles that make up the rotator cuff or the specific functions described in the scenario.
- **Option D:** This option might include muscles not part of the rotator cuff or omit crucial muscles like the supraspinatus.

## **Clinical Pearl / High-Yield Fact**
A key clinical test for rotator cuff injury is the **drop arm test**, where the patient is asked to hold their arm abducted to 90 degrees. Inability to hold this position or a slow, uncontrolled descent of the arm indicates weakness of the rotator cuff muscles, particularly the supraspinatus.

## **Correct Answer:** .