A 27-year-old male painter is admitted to the hospital after falling from a ladder. Physical examination reveals that the patient is unable to abduct his arm more than 15deg and cannot rotate the arm laterally. A radiographic examination reveals an oblique fracture of the humerus. He has associated sensory loss over the shoulder area. Which of the following injuries will most likely correspond to the symptoms of the physical examination?

**Core Concept**
The patient's symptoms of arm abduction and rotation limitation, along with sensory loss over the shoulder area, suggest an injury to the nerves supplying the arm muscles and skin. The humerus fracture indicates a traumatic injury to the surrounding nerves.

**Why the Correct Answer is Right**
The symptoms described are consistent with an injury to the axillary nerve, which is a branch of the brachial plexus. The axillary nerve supplies the deltoid muscle responsible for arm abduction and the teres minor muscle involved in lateral rotation. It also provides sensory innervation to the skin over the shoulder area. The axillary nerve is susceptible to injury in the setting of a humerus fracture, particularly an oblique fracture, as the nerve is closely associated with the bone.

**Why Each Wrong Option is Incorrect**
**Option A:** The radial nerve, which is also a branch of the brachial plexus, primarily supplies the extensor muscles of the wrist and fingers, and is not responsible for arm abduction or lateral rotation.

**Option B:** The musculocutaneous nerve, another branch of the brachial plexus, supplies the biceps brachii and brachialis muscles involved in elbow flexion, and is not associated with the patient's symptoms.

**Option C:** The ulnar nerve, which is a branch of the brachial plexus, primarily supplies the intrinsic muscles of the hand and is not responsible for arm abduction or lateral rotation.

**Option D:** The suprascapular nerve, which is a branch of the brachial plexus, supplies the supraspinatus and infraspinatus muscles involved in shoulder abduction and external rotation, but is not as closely associated with the humerus as the axillary nerve.

**Clinical Pearl / High-Yield Fact**
The axillary nerve is at risk of injury in any humerus fracture, particularly oblique fractures, due to its close proximity to the bone. A thorough neurological examination is essential in such cases to identify potential nerve injuries.

**Correct Answer:** C.

✓ Correct Answer: C. Fracture of the surgical neck of the humerus