WBR0558
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| 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 |
| 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.
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. |
| 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:: |