WBR0558

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz [1] and Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles
Prompt	[[Prompt::A 54-year-old man presents to the orthopedic surgeon with weakness and pain in his left shoulder. The pain began suddenly when the patient attempted to lift a heavy box one month ago. While the pain has largely subsided, the patient is still unable to initiate shoulder abduction. Physical examination of the left shoulder reveals decreased muscle bulk on the superior aspect of the scapula when compared to the right shoulder. MRI of the shoulder reveals a torn tendon. Which of the following areas of the brachial plexus gives rise to the nerve that normally innervates the injured muscle in this patient?]]
Answer A	AnswerA::Axillary Nerve
Answer A Explanation	[[AnswerAExp::While the axillary nerve innervates the deltoid (abducts the shoulder beyond the initial 15 degrees), the deltoid is not the most likely site of injury in this patient.]]
Answer B	AnswerB::Musculocutaneous Nerve
Answer B Explanation	[[AnswerBExp::The musculocutaneous nerve is responsible for innervating the biceps.]]
Answer C	AnswerC::Posterior cord
Answer C Explanation	[[AnswerCExp::The posterior cord gives rise to the axillary and radial nerves. While the axillary nerve innervates the deltoid, an important shoulder abductor, the deltoid is not the most likely site of injury in this patient.]]
Answer D	AnswerD::Union of C5-C6 nerve roots
Answer D Explanation	AnswerDExp::The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the superior trunk of the brachial plexus (C5-C6). All rotator cuff muscles are innervated by C5-C6.
Answer E	AnswerE::Union of C7-C8 nerve roots
Answer E Explanation	AnswerEExp::The union of the C7-C8 nerve roots represents the inferior trunk of the brachial plexus.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The supraspinatus muscle and tendon are commonly injured in rotator cuff injuries. The supraspinatus originates from the medial edge of the supraspinous fossa, where it proceeds to pass under the cover of the acromion to attach to the lateral aspect of the head of the humerus. The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the superior trunk of the brachial plexus (C5-C6). Several activities may injure the supraspinatus muscle and tendon, including trauma to the shoulder, lifting heavy objects, and repetitive arm movements (eg swimming). The rotator cuff is composed of 4 muscles: Supraspinatus, Infraspinatus, Teres minor, and Subscapularis (SITS). All 4 rotator cuff muscles are innervated by C5-C6. Supraspinatus: Causes initial arm abduction (initial 15 degrees). It is most commonly involved in rotator cuff injuries. Infraspinatus: Causes lateral rotation of the arm. Commonly injured in pitching injuries. Teres minor: Causes arm adduction and lateral rotation. Subscapularis: Causes arm adduction and medial rotation. Educational Objective: The supraspinatus is a rotator cuff muscle that is responsible for initiating shoulder abduction. All rotator cuff muscles are innervated by the suprascapular nerve, which originates from the C5-C6 roots. References: First Aid 2014 page 411]]
Approved	Approved::Yes
Keyword	WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Tendon, WBRKeyword::Rotator cuff, WBRKeyword::Rotator cuff injury, WBRKeyword::Rotator, WBRKeyword::Cuff, WBRKeyword::Supraspinatus, WBRKeyword::Infraspinatus, WBRKeyword::Subscapularis, WBRKeyword::Teres minor, WBRKeyword::SITS, WBRKeyword::Shoulder, WBRKeyword::C5, WBRKeyword::C6
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::