A patient had met with an accident and he can not abduct his right arm and can not lift it. On examination tenderness felt near right upper arm. X-ray showed fracture surgical neck of humerus. Muscle that was paralyzed was:
**Core Concept**
The surgical neck of the humerus is a common site for fractures, particularly in young adults, due to its thin cortical bone. The surrounding muscles and nerves play a crucial role in the movement and function of the arm.
**Why the Correct Answer is Right**
The deltoid muscle is primarily responsible for the abduction of the arm, and its paralysis would result in the inability to lift the arm. The deltoid muscle has a dual innervation, with the axillary nerve providing innervation to the middle and lateral parts of the muscle, and the suprascapular nerve providing innervation to the anterior part of the muscle. However, the axillary nerve is more closely associated with the function of the deltoid muscle. In the case of a fracture of the surgical neck of the humerus, the axillary nerve is often injured, leading to deltoid paralysis.
**Why Each Wrong Option is Incorrect**
* **Option A:** The rotator cuff muscles, including the supraspinatus, infraspinatus, teres minor, and subscapularis, are primarily responsible for stabilizing the shoulder joint and are not directly involved in arm abduction.
* **Option B:** The biceps brachii muscle is responsible for flexion and supination of the forearm, but not abduction of the arm.
* **Option C:** The trapezius muscle is responsible for elevation and downward rotation of the scapula, but not abduction of the arm.
* **Option D:** The latissimus dorsi muscle is responsible for adduction and extension of the shoulder joint, but not abduction.
**Clinical Pearl / High-Yield Fact**
A patient with a fracture of the surgical neck of the humerus may present with a classic clinical sign known as "drop arm sign," where the patient is unable to lift their arm due to deltoid paralysis.
**Correct Answer:** A. Deltoid.