WBR0558

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles
Prompt	[[Prompt::A 54-year-old male 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 unable to initiate shoulder abduction. Physical exam of the left shoulder reveals decreased muscle bulk on the superior aspect of the scapula when compared to the right shoulder. Ultrasound 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, an important shoulder abductor, 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. The muscle injured in this patient is the supraspinatus.]]
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).
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 patient in this scenario suffers from a torn supraspinatus tendon. The supraspinatus is the most commonly injured muscle in rotator cuff injuries. The supraspinatus originates on 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). Wiki-mnemonic for rotator cuff muscles = SITS Subscapularis Infraspinatus Teres minor Suprascapularis Educational Objective: The supraspinatus is responsible for initiating shoulder abduction and is innervated by the suprascapular nerve, which originates from the C5-C6 roots. References: First Aid 2012 page 405]]
Approved	Approved::Yes
Keyword	WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Shoulder, WBRKeyword::Arm, WBRKeyword::Upper limb, WBRKeyword::Rotator cuff, WBRKeyword::Tendon
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